Introduce `ContentType.objects.get_for_models(*models)` and use it in the the...

Introduce `ContentType.objects.get_for_models(*models)` and use it in the the auth permissions code.  This is a solid performance gain on the test suite.  Thanks to ptone for the profiling to find this hotspot, and carl for the review.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16963 bcc190cf-cafb-0310-a4f2-bffc1f526a37