Add tests and documentation to httptester app

I've started this refactor in the feature/httptester-update branch.

* Implementation improvements to be made:
    * Simplify view setup (just include httptester in your root URL conf)
    * Convert storage API from using cache to it's on database model. This gets around having issues related to multi-threaded environments and in-memory caches, setting up memcached, and making it easier to use in general.
    * Since we'll be storing messages in the database for a particular connection, we should have a way to flush/delete httptester messages for that connection in the web front-end.
    * Might as well update to latest Django norms, like redirect() and render()
* Add docs under ``docs/topics/contrib/handlers.rst`` with app overview.
    * Include installation procedure with ``INSTALLED_APPS``, ``INSTALLED_BACKENDS``, and updating ``urls.py``
    * Update ``docs/index.rst`` with proper link to new page.
* Add tests under ``lib/rapidsms/contrib/handlers/tests.py`` and make sure we have adequate test coverage. Test views, forms, and storage API.
* PEP8 fixes