Add Authentication Backend

[michaelwheeler]

It seems like it might be useful to create a Django Authentication Backend to allow apps to authenticate users based on LTILaunch information.

The backend might be something as simple as:

from django.contrib.auth.backends import BaseBackend

class LtiLaunchAuthenticationBackend(BaseBackend):
    def authenticate(self, request, lti_launch=None):
        return lti_launch.user.auth_user