Test and document contrib.registration

* [x] Add docs under ``docs/topics/contrib/registration.rst`` with app overview.
    * [ ] Include installation procedure, including the dependency on ``rapidsms.contrib.handlers``
    * [ ] Include description/example of both ``language`` and ``register`` keywords.
    * [ ] Update ``docs/index.rst`` with proper link to new page.
* [ ] Add tests under ``lib/rapidsms/contrib/registration/tests.py``
* [x] PEP8 fixes
* [x] Update to newer Django render(), etc., conventions
* [x] Remove ``README`` file and move any relevant content to docs.