Fixed #5461 -- Refactored the database backend code to use classes for the creation and introspection modules. Introduces a new validation module for DB-specific validation. This is a backwards incompatible change; see the wiki for details.

      git-svn-id: http://code.djangoproject.com/svn/django/trunk@8296 bcc190cf-cafb-0310-a4f2-bffc1f526a37