Add initial support for documentation via breathe/sphinx
Add doc/requirements.txt to install via pip
Use PACKAGE_NAME instead of _TARNAME for Doxygen
Define version etc. for Sphinx via Makefile.am

Using a conf.py.in does not work well, so all these variables
need to be set by autoconf during runtime.
Add "MIT license" to Sphinx copyright footer
Add comments about `needs_sphinx` version
Extend doc/index.rst and add initial doc/configuration.rst
Add GraphViz to Sphinx doc
Add and use Sphinx doc TODO notes
Rewrite doc index.rst project introduction
Merge pull request #14 from robo9k/breathe-doc

Add documentation via Sphinx/breathe