First step towards a MediaManager class: Compat one.

To get us moving towards a MediaManager class, the first
idea is to create a class that wraps our current dict based
manager and makes all users happy.

mediagoblin/tests/test_submission.py

@@ -231,7 +231,8 @@ class TestSubmission:
         media = self.check_media(request, {'title': u'Balanced Goblin'}, 1)
 
         assert media.media_type == u'mediagoblin.media_types.image'
-        assert media.media_manager == img_MEDIA_MANAGER
+        assert media.media_manager.mm_dict == img_MEDIA_MANAGER
+        assert media.media_manager.entry == media
 
 
     def test_sniffing(self, test_app):