refs #222 - Minor PEP8 changes and other compulsions

Conflicts:

	rapidsms/contrib/messaging/urls.py
refs #222 - Docs for rapidsms.contrib.messaging
Added empty models file for rapidsms.contrib.handlers
``coverage html`` would not run without it.
refs #222 - Tests and bugfixes for rapidsms.contrib.messaging
Listed django-selectable on README
Typo fixes
Thanks @dpoirier!
Merge pull request #279 from caktus/feature/messaging-update

refs #222 - Tests and docs for rapidsms.contrib.messaging