Fixed #6095 -- Added the ability to specify the model to use to manage a...

Fixed #6095 -- Added the ability to specify the model to use to manage a ManyToManyField. Thanks to Eric Florenzano for his excellent work on this patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@8136 bcc190cf-cafb-0310-a4f2-bffc1f526a37