Test and document contrib.messagelog

* Add docs under ``docs/topics/contrib/messagelog.rst`` with app overview.
    * Include installation procedure.
    * Describe how it works and the implications of enabling the app.
    * Update ``docs/index.rst`` with proper link to new page.
* Add tests under ``lib/rapidsms/contrib/messagelog/tests.py`` and make sure we have adequate test coverage
* Add initial south migration
* Update to newer Django render(), etc., conventions
* PEP8 fixes