Adds app_label to models/ directories

I think this was the cause of our heisenbug where the table names were
occasionally wrong, causing database errors.  I have no idea why that was not happening consistently, but this seems like it would cause it.  This adds a class Meta with an app_label per https://docs.djangoproject.com/en/1.4/ref/models/options/#app-label since we have a models directory instead of a models.py.  (This will no longer be necessary (although I think will not hurt) in django 1.7.)