Upgrade TranslationQueryset API with missing methods

Quick fixes, ensuring `TranslationQueryset` API is consistent with django's `QuerySet`.
- Imlement Django 1.6's `.earliest` method.
- Fix passing `None` to `TranslationQueryset.latest` (it now translated the name of the field specified on the model's `Meta.get_latest_by`.
- Make `bulk_create` raise a NotImplementedError instead of breaking.