Fix GAE backend to work on more recent versions of django-social-auth.

Initially GAEBackend.name did not match the name in BACKENDS, it worked fine.
In newer versions the names need to match in order to get loaded.

Note that this change in the name will affect the login url, since it's matched against the value in BACKENDS.
Merge pull request #606 from dragonx/master

One line fix to update Google App Engine backend to work with latest version of django-social-auth