Fixed #9448 -- `Layer` objects now carry a reference to their parent...

Fixed #9448 -- `Layer` objects now carry a reference to their parent `DataSource`.  Thanks, Matthew D. Hancher for the bug report.

Backport of r9284 from trunk.


git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.0.X@9285 bcc190cf-cafb-0310-a4f2-bffc1f526a37