Use module_name instead of object_name.lower()

This is a minor refactor to follow the Django paradigm. Using `module_name` has been standard practice for a while, at least [since 1.0](https://github.com/django/django/blob/1.0/django/db/models/options.py#L59-L60).

*Note:* this will change in 1.6 to `model_name`. See https://code.djangoproject.com/ticket/19689