CONTAINER BAG FOR FLUIDField of the Invention The present invention relates generally to the field of packaging, and more particularly, to the packaging of fluids for convenient storage, transportation and use. More specifically, the present invention relates to a novel beverage container having a built-in straw to easily consume the beverage. Background of the Invention The use of a flexible bag for storing fluids for convenient storage, transport and use is well known in the art. For example, these bags can be used to store medical fluids for intravenous infusion to a patient, or for purified water or saline in a first aid kit. These bags are also used together with drinks designed for human consumption. When used as a container, the problem arises of how to open the package and then use the content without spilling it. A common solution in the beverage market is to provide a straw or straw wrapped with each bag, fixed on the outside of the bag. These straws are usually stiff and have a sharp tip at one end. A weak point of the bag, predetermined and appropriately designated, is punctured with the sharp tip of the straw allowing consumption of the contents. This provision has certain disadvantages. For example, the straw can be separated from the bag before its intended use. For reasons of durability and preservation of the content, called barrier properties, said bags are typically flexible and difficult to open by hand tearing them apart. In that case, the package could be opened only with great force and probably spilled from the content in the process. Therefore, it would be inconvenient to consume the content if the straw was separated. Even if it is not separated, the straw wrapping can be damaged, compromising its hygiene and that of the contents when the straw is inserted in the bag. The U.S. patent No. 3,799,914 issued to Schmit, et al., Describes a fluid bag having a straw enclosed therein. However, to prevent the fluid from flowing through the straw into an upper chamber, the straw must be sealed by means of a complex bending procedure. This would require labor or automated machinery to perform this task. In addition, the lower chamber should be sealed around the straw. In production, it would be common for the straw to be sealed irreversibly in the process, for example, when heat is applied to seal the bag around the straw as described. Then, the straw and, therefore, the bag, would be unusable. Other beverage bags known in the art are shown, for example, by Darmstadter, U.S. patents. Nos. 5,941,642 and 6,220,756-B1. Darmstadter shows the inclusion of a straw inside the bag; however, the bag has only one camera and the straw is merely a straight straw, not a flexible straw. Consequently, it extends very little beyond the bag when it is opened and, therefore, it is relatively inconvenient to use. Another beverage bag containing a straw is shown by Christine, et al., U.S. Pat. No. 5,564,591. Again, this straw is straight, not flexible, and extends very little beyond the bag when it is opened. Also, its use is inconvenient. Moreover, with respect to Christine, the bag requires that the straw be broken before use and the bag also requires a sealed accessory at an upper end of the straw. SUMMARY OF THE INVENTION In order to overcome these and other difficulties in the prior art, a fluid container bag having a flexible straw incorporated for drinking or dispensing is described. A fluid containment bag according to the present invention comprises at least first and second fluid chambers having at least two flow channels for fluid communication between the first and second fluid chambers. At least one of said flow channels comprises a flexible communication tube, such as a flexible drinking straw. According to another aspect of the present invention, a bag for fluid drinks is formed between opposed collapsible side walls made of a material consisting of a layer of polypropylene having a barrier layer on at least one side. In accordance with the present invention, a fluid container bag is provided which is provided internally with a flexible straw. This allows a convenient and hygienic consumption of the drink. This goal is also achieved while preventing the fluid from collecting in an upper chamber where a distal portion of a flexible straw remains without the need to seal the straw in the packing process, nor the risk of permanently sealing accidentally pop the straw in the packaging process. Furthermore, according to the present invention, a beverage container bag is provided which achieves adequate barrier properties compared to the materials currently used for such purposes at a significantly reduced cost. According to another embodiment of the present invention, a fluid containing bag has first and second opposed side walls forming at least one closed chamber between them, each side wall consisting of a laminated material having at least two layers. A first layer of the laminate is a sealing layer for sealing with a sealing layer of an opposite side wall. A second layer of the laminate provides a barrier between the contents of the bag and its environment. Each of the first and second layers comprises a material having an oriented structure, so that at the sufficient energy to tear the material is directed in a predetermined direction through the material; and further, that the oriented structures of the first and second layers are aligned with each other. At least a weakening of design in the bag is operative to direct the tearing energy in a direction aligned with the oriented structure of the first and second layers. BRIEF DESCRIPTION OF THE DRAWINGS These and other features and advantages will be apparent from the following descriptions and drawings, where similar numerals designate similar characteristics in all figures. Figure 1 shows a prospective view of a container bag for full fluid according to the present invention; Figure 2 shows a side elevational view of the filled bag; Figure 3 shows a front elevation view of a container bag for empty fluid according to the present invention; Figure 4 shows a side elevational view of an empty fluid bag; Figure 5 illustrates the flexible drinking straw used in conjunction with an exemplary embodiment of the present invention; Figure 6 shows a perspective view of a bag according to the present invention after opening; Figure 7 shows a sectional view taken along line 7-7 of Figure 6; Figure 7A shows the portion of Figure 7 within the circle 7A, 7B according to one embodiment, enlarged and in further detail; Figure 7B shows the portion of Figure 7 within the circle 7A, 7B according to an alternative embodiment, enlarged and in further detail; Figure 8 shows a modality of the bag bearing distinctive marks, shown against a dark background to highlight the contours of the bag; Figure 9 shows a cross section in the upper portion of the bag according to another embodiment, taken along the line 9-9 of Figure 3, including a resealable closure. Detailed Description of the Invention Referring now to Figures 1 and 2, an exemplary embodiment of a full fluid containing bag, generally 10, is illustrated. The bag has an upper portion 12, two side portions 14, 16 and a lower portion. 18. The first fluid chamber 20 and the second fluid chamber 22 are provided between the opposite collapsible walls 24 and 26. The first fluid chamber 20 and the second fluid chamber 22 are generally separated by a margin 12a formed in the portion upper 12. A flexible communication tube, for example, a flexible drinking straw 28, is provided inside the bag 10 between the first chamber 20 and the second chamber 22, forming a first flow path between them. The flexible straw is not sealed and provides a first flow path 29 that allows fluid communication between the first chamber 20 and the second chamber 22. A second flow path 30 also allows fluid communication between the second chamber 22 and the second chamber 22. first chamber 20. In an exemplary embodiment, the second flow path 30 is coaxial with the flexible straw 28 and surrounds it. In an alternate embodiment, the second flow path 30 is a one-way flow path that allows flow only from the second chamber 22 to the first chamber 20. One more design weaknesses, e.g., tear notches 32a and 32b, are provided in the upper portion 12 to allow the bag to be opened by hand by tearing it open. One or more through holes 34a and 34b are provided in the upper portion 12 to allow the bag to be hung from an exhibitor, for example, in a retail store aisle or a refrigerated container. A portion of the bag, the upper portion 12 in the example of Figure 1, can be formed in a non-rectangular manner as defined by the edges 35a, 35b. One method of doing this would be a die cutting press, other methods are known in the art. These cuts can be varied to fit the distinctive markings printed on any of the walls 24, 26 of the bag or both. With reference to Figure 8, in an exemplary embodiment, the distinguishing marks 110 include a scene having mountains with clouds above the mountains. The distinguishing marks 110 are printed on at least one wall and the upper portion 12 of the bag 10 is die cut to the shape of the clouds. The notch 32b can be formed to match the distinctive marks by a die cut between a cloud on top and a slope on the mountain below. Referring now to Figures 3 and 4, the construction of the bag will be described in more detail. The opposite collapsible walls 24 and 26 can be formed by either joining two sheets of a suitable material and sealing them along the upper portion 12, both side portions 14, 16 and the lower portion 18. This can be carried out either by heat sealing or any other suitable means known in the art. The flexibility of the foldable wall material will allow the first chamber 20 and the second chamber 20 to be formed between the opposite collapsible walls 26 and 28 when filled with a liquid. An alternative and preferred construction is to fold a single sheet of the desired bag material onto itself and seal along the upper portion 12, and both side portions 14, 16. More preferably, whether it is formed of two separate sheets or a single folded sheet, the lower portion 18 is provided with at least one reinforcement 36 to allow expansion for additional fluid capacity, as well as to reinforce the seal at the lower portion 18. Additionally, either side portions or both they can be provided with reinforcements independently of any reinforcement 36 in the lower portion. The bag 10 can be constructed from a single layer material or a multilayer material. Materials commonly used for beverage bags include a polyester layer sandwiched between layers of aluminum foil on either side. Although aluminum foil is generally considered an excellent barrier, it has certain disadvantages. Among these, when the aluminum sheet is flexed, microscopic and macroscopic cracks easily develop on the surface, which severely compromises the barrier properties. Also, it is extremely difficult to tear the aluminum sheet by hand and, if it can tear, the direction of the tear proceeds at random and requires other opening methods as described above. Referring now to Figure 7A, a more preferred material that would allow for reduced costs without sacrificing barrier properties is a layer of polypropylene film 101 having an oxygen barrier layer 103 on only one side. The oxygen barrier layer 103 may be a metallized coating, an aluminum foil layer, a polyethylene vinyl alcohol layer or other collapsible material with suitable oxygen barrier properties known in the art. This combination provides significant advantages. The oxygen barrier layer 103 complements the oxygen barrier properties of polypropylene. This is particularly important for carbohydrate-rich beverage liquids, where exposure to oxygen during shipment and storage adversely affects the quality of the content, including the convenience of consumption. In addition, the above barrier properties can be obtained at a reduced cost with respect to the polyester material sandwiched between layers of aluminum foil as commonly used. With reference to Figure 7A, the bag 10 is preferably formed by orienting the oxygen barrier layer 103 outwardly. The outward facing barrier layer allows the superior moisture barrier properties of the polyethylene layer 101 to prevent contact between the fluid content of the bag and the oxygen barrier layer, which could compromise the properties of the oxygen barrier layer 103. The bag 10 can then be formed by folding a single sheet of such material in the manner described above. The choice of barrier layer materials will be influenced by aesthetic issues without departing from the scope of the present invention. For example, a bag having a layer of metalized oxygen barrier or aluminum foil facing outwards will have a reflective outer surface, which would allow attractive and showy distinctive markings to be applied. The use of polyethylene vinyl alcohol for the oxygen barrier has the advantage of transparency, which could be combined with a transparent polypropylene layer to make the contents of the package visible. In a more preferred embodiment, illustrated in Figure 7B, a multilayer laminate comprises a deeper sealant layer 150. The sealant layer may comprise polyethylene, but other materials may be suitable. Polyethylene has the advantage of a low melting point. Therefore, two sheets of the side wall material can be joined with respective sealing layers towards each other. Heat can be applied from an external source, at a temperature above the low melting point of the polyethylene, but below that of the other layers. Thus, the sealing layers fuse together, forming an airtight seal in the region 152. The seal 152 exists only in the margins of the bag 10 and the sealing layer 150 is open to the first and second chambers 20, 22 where it is not seal form. In a more preferred embodiment, polyethylene is linear low density polyethylene (LLDPE). The LLDPE is stiffer and has improved tear characteristics, as described below. In another embodiment, heat can be applied to the opposing sealing layers 150 before the two are joined. Other methods of sealing, including but not limited to bending, or application of solvent or adhesive can be substituted. The preferred laminate further comprises a barrier layer 154, which is bonded to a sealant layer 50 by means of a layer of adhesive 156. The barrier layer 154 may comprise multiple layers. In the prior art, suitable barriers included an aluminum foil layer. Polypropylene, while presenting other desirable qualities, is not commonly considered to have acceptable oxygen barrier properties. However, recent advances in material technology have developed a mixed polypropylene material, which includes an aluminum layer deposited under vacuum as part of a laminated material. An example of such material is marketed under the trade name OPPALYTE by ExxonMobil Chemical Corporation, more specifically designated Ultra-High Barrier 35 MU 842. The material MU MU 842 provides a significant barrier to the permeation of oxygen, water vapor and flavor or aroma , previously not available in a polypropylene material. Furthermore, in comparison with other materials having similar barrier properties, for example, a mixed sheet and polyester material discussed above, the material MU 842 retains its barrier properties after being subjected to bending and / or bending. Outside the barrier layer 154, an additional adhesive layer 158 is bonded to an outer layer 160. The outer layer 160 may comprise polyester, PET, oriented polypropylene, polyethylene or LLDPE, among other materials. The outer layer 160 may be present to provide resistance to additional perforation or other protection of the generalized content, depending on the selection of the other materials. In alternate embodiments, outer layer 160 and sealing layer 154 may be omitted. The outer layer 160 may have an inverted print on its inner surface to show distinctive markings to its outer surface, in a manner known in the art. The precise choice of materials will depend on the particular application, and more specifically, the intended content of the bag and the barrier properties that are required. For example, an oxygen barrier is desirable for a carbohydrate-rich beverage prepared without preservatives. In another contemplated application, the bag will contain one or more doses of a liquid pharmaceutical product for medical supply. Many medications, for example, a cough medicine, contain significant amounts of alcohol. Therefore, the bag material must present a significant barrier to at least alcohol and moisture. However, such medications are often preserved for a prolonged storage life, so the oxygen barrier is not a cause for concern. In that case, other barrier materials may be provided in place of, or in addition to, an oxygen barrier material. These include, but are not limited to, nylon, polyester, polystyrene, fluoropolymers and ethylvinyl alcohol, among others. Each of the named materials maintains an orientation and tearing ability in accordance with other aspects of the present invention. Other contemplated applications of the present invention include, but are not limited to, the dosage and application of pesticides. The particular pesticide in question will present needs for particular barrier properties, which will guide the selection of material, without departing from the scope of the present invention. Referring now to Figures 1, 6 and 7, the use of the beverage container bag will be described. During transport and handling, the contents can flow via the flexible straw 28 between the first chamber 20 and the second chamber 22. However, it is not advisable for the fluid content to collect in the second chamber when the bag is opened. For this purpose, the second flow path 30 allows any fluid directed to the second chamber 22 via the flexible straw 28 to be recirculated back to the first chamber 20 under the influence of gravity via the second flow path 30 when the package it is placed in a vertical orientation, for example, when it is opened. To assist in this recirculation, a lower edge 40 of the second chamber 22 can be moved at an angle to divert any fluid within the second chamber 22 to the second flow path 30 and then to the first chamber 20. To open the bag, a user would tear the upper portion 12, starting at a designated weakening, for example, the notch 32b. Continuing in the direction of the arrow 100, the second chamber 22 would be opened along the tear line 204. The flexible straw 28 may be deflected in such a way as to cause it to rise in the direction of the arrow 102 once. the chamber 22 is free of any restrictions downwardly of the upper portion 12. Therefore, one end of the flexible straw 28 close to the user rises markedly above the top of the bag 10. The user can then consume the drink or use the content in another way. In the prior art it is a problem that the characteristics of a bag that make it flexible also make the bag difficult to open to use or consume the contents. For example, aluminum foil, commonly used in prior art bags, is particularly resistant to puncture or tearing. Therefore, according to one aspect of the present invention, the components of a multilayer laminate have an oriented structure. More specifically, these oriented structures are aligned parallel to each other and to the tear line 204. The tear line 204 is shown horizontally to the bag 10, but may traverse the bag at any angle. The tear notch 32b concentrates the tear energy stress along the tear line 204 and together with the parallel aligned orientation of the material structure, the tear advances along the tear line 204 and through the bag. Furthermore, it is even more preferred to cause the tear line 204 to cut transversely an open portion of the at least one chamber, in this case the second chamber 22. In this way, as the tear advances along the line of tear 204, the user only needs to tear two single layers for at least a portion of the tear, which has less strength than the margin portion where the two opposed layers are sealed together. In another embodiment of the invention, with reference to Figure 9, the bag 10 is provided with a resealable closure 202 in the second chamber 22. The resealable closure 202 may be of the tongue and groove variety, wherein the seal is formed by the abutting surface of a tongue portion 202a and a slot portion 202b. Other resealing means known in the art may also be suitable. The resealable closure 202 is provided below the tear line 204 defined by the notch 32. In the case where in the flow path 30 is only a one-way flow path, the bag can be inverted and / or tight for your content. A further advantage of the present invention is that the provision of the straw inside the bag, particularly through the diagonal dimension of the bag as shown in the figures, improves the structural stability of the bag in all dimensions , particularly during handling and transport. The invention has been described herein with reference to particular exemplary embodiments. Certain alterations and modifications may be apparent to those skilled in the art, without departing from the scope of the invention. The exemplary embodiments are not intended to limit the scope of the invention, which is defined by the appended claims.