Oauth authentication

OAuthAuthentication class as discussed in #8.

It usues `django-oauth-plus` and `oauth2` packages. This backend is optional because strongly depends on external libs. This is a reason I don't add new packages to `requirements.txt`.

Added new tests, Auth class and minimal documentation. I changed travis configs too.

In next days i will provide more detailed documentation but at first I will try to add more structure for existing [django-oauth-plus docs](http://code.larlet.fr/django-oauth-plus/wiki/Home).