Sphinx Plugin

This relates to #98 which was a feature request for a Sphinx plugin. It includes changes as per the feedback received on my old dev/sphinxPlugin branch.

The plugin right now is very basic but will hopefully still be useful. I would hope to add some additional functionality in the short term. You'll see here http://sphinx-doc.org/invocation.html for example that sphinx-build offers a myriad of options, some of which there would be a strong case for supporting. Sphinx also has the sphinx-apidoc command which automagically generates rst files for entire projects. Could also be a useful addition and would be used together with the sphinx_generate_documentation task the plugin already exposes.