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