Fixed #3257 -- Added newforms ModelChoiceField and ModelMultipleChoiceField, which are now used by form_for_model() and form_for_instance(). Thanks for the patch, Honza Kral, floguy@gmail.com and kilian.cavalotti

git-svn-id: http://code.djangoproject.com/svn/django/trunk@4547 bcc190cf-cafb-0310-a4f2-bffc1f526a37