Show inheritance tree of testing harness classes

The <a href="http://rapidsms.readthedocs.org/en/latest/topics/testing.html">testing docs</a> don't cover the complex inheritance tree of the testing mixins - for example, that ``CustomRouterMixin`` is not separate from ``CreateDataMixin`` but actually inherits from it. This is confusing and should probably be clarified.

@dpoirier pointed out that we can make more use of autodoc to show inherited classes.