Allow .id instead of ._id for the Mongo backend

To allow easier migration to the SQLAlchemy style .id give
the User and MediaEntry mongo classes an alias attribute of
.id that maps to ['_id'].

Use it in the upload process, because this was one of the
last positions with a ['_id'] instead of ._id (due to a bug
in mongokit).

mediagoblin/submit/views.py

@@ -59,7 +59,7 @@ def submit_start(request):
 
                 # create entry and save in database
                 entry = request.db.MediaEntry()
-                entry['_id'] = ObjectId()
+                entry.id = ObjectId()
                 entry.media_type = unicode(media_type)
                 entry.title = (
                     unicode(request.POST['title'])