Initial stab at writing docs

Used sphinx so the docs could be hosted at Read the Docs.

Closes #41 and #54 .
Bizarrely, `raise (ValueError, TypeError), "foo"` works and appears to be equivalent to `raise ValueError("foo")`. The [docs for raise](https://docs.python.org/2/reference/simple_stmts.html#the-raise-statement) don't mention this, and I can't think what it's useful for.