Convert nacl.public docs

Ok, so here's a start to the docs fixing effort I mentioned in #52.

1. The first commit adds the rtd theme to the tox build so I can get a consistent idea of how it will look on the actual site.

2. I'm going to deal with this move on a module basis to keep the PRs reasonably small and reviewable. The second commit in this PR deals with the `nacl.public` module. 

Some things to note,

1. What does the `Box.decode()` class method do? It isn't documented prior to this. (Fixed)
2. `nacl.utils.EncryptedMessage` will temporarily not be documented once this gets merged because I am moving the `nacl.util` docs into a separate page.
3. I'll need sanity checks on things like typos and types.

@warner @dstufft, for your reviewing pleasure. :)