Fixed #7560 -- Moved a lot of the value conversion preparation for

loading/saving interactions with the databases into django.db.backend. This
helps external db backend writers and removes a bunch of database-specific
if-tests in django.db.models.fields.

Great work from Leo Soto.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@8131 bcc190cf-cafb-0310-a4f2-bffc1f526a37