Improve the documentation

Your project is so nicely implemented and clearly commented, I thought it deserves an improved documentation.

Besides, other great projects like [`django-rest-framework`](https://github.com/tomchristie/django-rest-framework) are willing to use yours as a dependency for the OAuth 2 support, but to achieve that it really needs a fresh-and-stylish documentation :)

I've made the changes necessary to remove all documentation build warnings.
And now the doc will be able to have this [beautiful new design](https://django-oauth2-provider-dulaccc.readthedocs.org/en/latest/). This is a just temporary (and protected) doc to show you how I've changed it.