Moved the settings of db_table to Options.contribute_to_class().

Some fields need to know the right db_table setting in their own
contribute_to_class(), so waiting until Options._prepare() is a little
inconvenient. This is a deep-internals change. No effect on external code.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@7777 bcc190cf-cafb-0310-a4f2-bffc1f526a37