Add tests for OAuth2 authentication
Add django-oauth2-provider to the installed apps
Add django-oauth2-provider to optionals.txt
Add the OAuth2Authentication class
Add OAuth2Authentication documentation
Merge branch 'master' into oauth2-authentication

Conflicts:
	rest_framework/tests/authentication.py
Improve the `django-oauth2-provider` import block

to avoid naming collision with `oauth2` used for OAuth 1
Use the correct doc link style
Clean up some print and comments
Add django-oauth2-provider to .travis.yml
Use django.utils to import the unittest module

for a cross python versions compatibility
Add new OAuth2 tests
Properly fail to wrong Authorization token type
Use the PyPI django-oauth2-provider version
Only add the django-oauth2-provider apps if the module is installed

otherwise log a warning
Update the documentation

with a warning for incompatibility with Python 3
and taking @tomchristie advice into account on how to reformulate some
sentences
Fix import errors
Update the package dependency url style in tox.ini
Clean ups.
Merge OAuth2 work.