Initial stab at writing docs

Used sphinx so the docs could be hosted at Read the Docs.

Closes #41 and #54 .
We should be able to subclass 2to3's fix_raise (though we don't currently - I'll make a separate issue for that), in which case we get this for free. Since someone thought it was worth supporting in 2to3, I wouldn't go to extra lengths to make it not work. I might leave it out of the docs as unnecessary confusion, though.