Django REST Framework 3.0 API changes

This PR adapts to the breaking changes in Django REST Framework 3.0.

The DRF model serializers have been simplified a lot and therefore some functionality (e.g. `pre_save`, `post_save`) had to be dropped. `BulkListSerializer` is a similar implementation to the example of a `list_serializer_class` in the [docs](http://www.django-rest-framework.org/api-guide/serializers/#customizing-multiple-update). However, `BulkSerializerMixin` has to be used in addition, as the `id` field is usually discarded.