Fixed #1662 -- Added resolver for string-form model references for models that...

Fixed #1662 -- Added resolver for string-form model references for models that have already been loaded, with tests to validate both forward and backward referenced model names. Light refactoring of model loading to make regression tests behave more like normal model loading. Also clarifies the text of some validation errors.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@3195 bcc190cf-cafb-0310-a4f2-bffc1f526a37