Make model save and create methods more customizable in serializers

I often use the `update_fields` kwarg of the model instance's `save()` method. When switching to DRF 3 serializers make it a bit harder than before: I would need to override and copy the code of `serializer.create()` and serializer.update()`

This patch makes it easier and more DRY: just override the single `create_instance` and `save_instance` serializer methods.