Fixed #5390 -- Added signals for m2m operations. Thanks to the many people...

Fixed #5390 -- Added signals for m2m operations. Thanks to the many people (including, most recently, rvdrijst and frans) that have contributed to this patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@12223 bcc190cf-cafb-0310-a4f2-bffc1f526a37