Some suggested code to add support for IPv6 to the IPAddress class. (Issue#168 )

The public interface changes are minimal:

• add a type() getter and corresponding enum
• add three new constructors: one for 16 octets, empty with type specifier, and byte pointer with type specifier
• add constant for IP6ADDR_ANY (similar function as IPADDR_NONE)

The changes are backward compatible. All of the IPv4 test cases continue to pass without any changes. i.e. Any IPv4 code will continue to get the same outputs for a given inputs.

Test cases added to cover IPv6.

Note: Whilst backwards compatible, this change does increase the memory footprint from 4 bytes to 20 bytes (assuming the enum compiles as an underlying int32_t, I think).