Initial stab at writing docs

Used sphinx so the docs could be hosted at Read the Docs.

Closes #41 and #54 .
I might look into using intersphinx for these links. It looks like the Python docs also add a `2to3fixer` directive, which I might steal, to make the fixers individually referenceable. That needn't be done in this PR, though - just getting docs there is valuable enough.