Provide easy customization of RestAdapter.

- Add `newRestAdapterBuilder()` to allow RestAdapter customization.
- Docs for Trakt class.
- Renamed `buildRestAdapter()`to `getRestAdapter()` to make clear that this always returns the current instance.
Prepare version 3.3.0.
Fix line breaks in README.
Update okhttp, okhttp-urlconnection to 2.0.0, make them optional.
Change log for 3.3.0.
Require Java 1.7.
Replace self-closing `<p/>` with `<p>`.
Merge branch 'release-3.3.0'