Fixed #7596. Added Model.objects.bulk_create, and make use of it in several...

Fixed #7596.  Added Model.objects.bulk_create, and make use of it in several places.  This provides a performance benefit when inserting multiple objects.  THanks to Russ for the review, and Simon Meers for the MySQl implementation.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16739 bcc190cf-cafb-0310-a4f2-bffc1f526a37