Doxygen+Sphinx+Breathe = Inline Documentation#582

This is the initial attempt at getting inline documentation in theproject implementation into the sphinx-generated documentation. The waywe achieve this is by doing the following:  - Add michaeljones/breathe as a submodule, making it simpler to    generate the documentation with just Sphinx installed on the    developer machine.  - Define a Doxyfile configuration which generates HTML and XML    documentation. The generated XML files are needed by Breathe to    generate the HTML as part of the Sphinx docs generation process.  - Update the documentation for the client type. This is a starting    point showing how the generated docs are in-lined in the RST docs    via breathe.  - Wire up the documentation generation in the Sphinx configuration.To generate the documentation now, we need two things installed on themachine generating the docs: Sphinx and Doxygen. The steps involved are:  1. cd cpp-netlib && doxygen  2. cd libs/network/doc/ && make htmlThe results should be in libs/network/doc/_build/html, and thegenerated HTML for the HTTP Client reference page should include thedoxygen-generated docs.