Fixes depreciation warnings in Django Auth.

Closes #156

mongoengine/django/auth.py

@@ -99,6 +99,10 @@ class MongoEngineBackend(object):
     """Authenticate using MongoEngine and mongoengine.django.auth.User.
     """
 
+    supports_object_permissions = False
+    supports_anonymous_user = False
+    supports_inactive_user = False
+
     def authenticate(self, username=None, password=None):
         user = User.objects(username=username).first()
         if user: