added a hook to allow a field to resolve any necessary dependencies before it's attributes are accessed during sql construction
added a hook to allow a field to resolve any necessary dependencies before it's attributes are accessed during sql construction

--HG--
extra : convert_revision : svn%3A69d324d9-c39d-4fdc-8679-7745eae9e2c8/trunk%4036

philipn/django-south,RaD/django-south,nimnull/django-south,nimnull/django-south,RaD/django-south,philipn/django-south,RaD/django-south