add tests for OAuth authentication
add django-oauth-plus & oauth2 to installed apps in runtests settings.py
add OAuthAuthentication class
add django-oauth-plus & oauth2 to .travis.yml
add django-oauth-plus & oauth2 to optionals.txt
add OAuthAuthentication documentation stub
Move oauth2 and django-oauth-plus imports to compat and fix some minor issues

- alias oauth2 as oauth
- remove rouge print
- remove docstring markups
- OAuthAuthentication.authenticate() now returns (user, token) two-tuple on success
- don't set request.user because it's already set
OAuth tests now are skipped unless django-oauth-plus and oauth2 are installed.
runtest.settings fixed if django-oauth-plus or oauth2 are not installed

oauth_provider can be added to INSTALLED_APPS only if these packages are installed
oauth2 & django-oauth-plus installed only on 2.x
import compat version of unittest
rfc5849 link with anchor
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