Remove register_backend() and use django's backend list instead.

Hi Matt, 

This change removes the 'register_backend' function and corresponding 'registered_networks' array, and instead loops through django's list of registered backends when trying to find the appropriate backend that matches the right social network.

I think this is a good change for two reasons:

1) It lets us create our own backend classes that inherit the network backends defined in django_social_user without worrying about collisions in the registered_networks array.

2) Network backends not added to AUTENTICATION_BACKENDS will 404 when you attempt to access their urls.  Currently (without this change), if you don't add a backend like TwitterBackend to AUTHENTICATION_BACKENDS, it's url will still resolve (authenticate/twitter/ and callback/twitter/ as defined in urls.py).

Thanks,
Matt Haigh