WIP Base implementation

Standalone edx-platform oauth2-provider. WIP still has a few issues to be solved:

- `SCOPE` definition is a bit wonky. `django-oauth2-provider` specs them to be define in the django settings under `OAUTH_SCOPES`. However, since this is a standalone package customized for the edx-platform, we want them to be constant. The file `constants.py` tries to do this by overwritting the constants in `django-oauth2-provider`. Several forms also had to be subclassed to override the definition of the `scopes` field, which uses the original values and are defined during import of `django-oauth2-provider`.

- setup.py should include a dependency to the edx/django-oauth2-provider fork, using dependency links as indicated here: http://mike.zwobble.org/2013/05/adding-git-or-hg-or-svn-dependencies-in-setup-py/

- there is no need to use the value `ENABLE_OAUTH2_PROVIDER` anymore.

- there should be a handler specified in the django settings, and implemented in the edx-platform, that responds to scope/grant queries using the corresponding interfaces in the platform. The idea is to decouple the provider from the platform.

- there should be integration tests in the edx-platform.

- the repository should have CI.
This may pose a problem for those using custom user models. Not an issue at the moment, however.