[Needs feedback] Sphinx Documentation

I started making documentation today. I would appreciate any feedback. The docs rendered from this PR are currently sitting at http://stanford.edu/~rmcgibbo/mixtape/

cc @schwancr, @rbharath @kyleabeauchamp, @mpharrigan, @gkiss, @tjlane, @mlawrenz, @VijayPande, @cxhernandez, @sryckbos, @leeping.