Moved OAuth support out of DRF and into a separate package.

Closes #1767.

This work came from a sprint at [DjangoCon](http://www.djangocon.us/) 2014.

This PR removes all functionality and tests for OAuth 1.0a and 2.0 from DRF (see #1767). The same functionality has been ported to the [Django REST Framework OAuth](https://github.com/jlafon/django-rest-framework-oauth) package. I plan on updating the DRF OAuth package to use [oauthlib](https://pypi.python.org/pypi/oauthlib), which is actively maintained and supports Python 3.

* Only necessary changes were made in moving code from DRF to DRF OAuth
* Dependencies in travis, tox, and requirements.txt for django-oauth-plus, oauth2, and django-oauth2-provider have also been removed
* The documentation has been modified to reference the 3d party package. More documentation changes are necessary, but not until after I update DRF OAuth as described above