Read The Docs! (kind of)

Spent some more time fiddling with RTD... this gets us a little closer. This solves the initial issue (infinite recursion) I reported in #87. Turns out there were a couple other users that had this issue as well (it's a bug in Sphinx 1.2).

Remaining issues:

- "Fork me on GitHub" banner not displaying. See [this RTD issue](https://github.com/rtfd/readthedocs.org/issues/152) for some workarounds. This is not a blocker.
- If the docs are rebuilt too soon after a previous build (not sure exactly what the time interval is), parts of the site won't build/render correctly because `make clean` needs to be run beforehand. I added a comment to [this RTD issue](https://github.com/rtfd/readthedocs.org/issues/208) to see if there's a workaround (other sites that use a similar numpydoc/autosummary/autodoc setup have reported this issue as well). This is a blocker.

I don't think it hurts to have this merged now, but we can't fully switch over to using RTD yet.