Compatibility with Django1.7

Several modifications in the Django1.7rc1 impact the usage of this lib:
- admin login redirection strategy: https://docs.djangoproject.com/en/dev/releases/1.7/#admin-login-redirection-strategy
- app-loading: https://docs.djangoproject.com/en/dev/releases/1.7/#standalone-scripts

This last point might need some clarification in the docs: in the Django docs,
it says that now `django.contrib.admin` autodiscovers automatically, and thus
the `admin.autodiscover` is now useless.
In fact, this is because it ships with an `AppConfig` that runs the
autodiscovery when the app-loading is done.
Unless you plan to have the same functionality in this lib, we still need an
explicit `autodiscover`.

Source: https://docs.djangoproject.com/en/dev/releases/1.7/#app-loading-refactor