prefer bulk_create to repeated create

Prefer bulk_create for adding multiple items to a sortedm2m. As sortedm2m relies on model for count of items, use transaction.atomic() around use of count value with Django 1.6 or later.
Instead of `self._item_and_count(new_ids)` you could use `enumerate(new_ids, 1)`.
And I think that `for count, item in ...` is more readable than using the tuple indices in the code then.