Fixed #7503 -- Allow callables in list_display. This also does a lookup on the...

Fixed #7503 -- Allow callables in list_display. This also does a lookup on the ModelAdmin for the method if the value is a string before looking on the model. Refs #8054. Thanks qmanic and Daniel Pope for tickets and patches.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@8352 bcc190cf-cafb-0310-a4f2-bffc1f526a37