Add Sphinx documentation

Add source-level documentation using http://sphinx-doc.org/ - no semantic or code changes in this patch 

1. Install Sphinx: pip install Sphinx 
2. build docs: make -C sphinx html
3. view docs: firefox spinx/_build/html/index.html

Docs can be hosted on http://readthedocs.org/ (though I'd like to finish some more patches first). Sphinx isn't included in requirements.txt b/c it's an optional build-time dependency (more soon)