Python package

Re: #13

Testing at http://tmuxp.readthedocs.org/en/latest/ (https://github.com/tony/tmuxp/blob/master/doc/requirements.pip)

I believe that @ericholscher will be doing the pypi, so we need communication for how to test this.

Unlike [sphinx-contrib](https://bitbucket.org/birkenfeld/sphinx-contrib/) extensions, themes don't have a best practice. So I'm playing by observation and practicality:

For a python package, I looked at https://github.com/ryan-roemer/sphinx-bootstrap-theme's ``__init__.py``. Sphinx-boot-strap themes, and other newer sphinx themes, including https://bitbucket.org/ecollins/cloud_sptheme, let the package assist with the ``html_theme_path`` by providing a function.

Bike-shedding: I think it's best to let the method return a string to be placed *inside* a theme path list. The path is one of many, and people unfamiliar with sphinx would otherwise have to assume that ``html_theme_path`` is a list and ``.append`` it.