moving SQL out of the core code. Only direct fields and foreign keys can be
updated in this fashion, since multi-table updates are very non-portable.
This also cleans up the API for the UpdateQuery class a bit. Still need to
change DeleteQuery to work similarly, I suspect.
Refs #4260.
git-svn-id: http://code.djangoproject.com/svn/django/branches/queryset-refactor@7043 bcc190cf-cafb-0310-a4f2-bffc1f526a37
