Add sphinxcontrib-django to docs extensions

[mfosterw]

Reasoning:

Right now the reference docs for models do nothing but list the fields while providing a generic description of a django field, and you can only really figure out what the fields do from the admin or by reading the source code. sphinxcontrib-django is an extension for autodoc which automatically documents django models based on their help_text. As you can see, before:

(ugly, useless) vs after:

(beautiful, informative). While I think the examples are good, there are some fields which aren't mentioned there that are nice to have described in the api reference. Obviously in order to provide the image reference I had to implement this, so I will open a pull request so others can check it out and browse the docs generated with the extension.