Fixed #4412 -- Added support for optgroups, both in the model when defining choices, and in the form field and widgets when the optgroups are displayed. Thanks to Matt McClanahan <cardinal@dodds.net>, Tai Lee <real.human@mrmachine.net> and SmileyChris for their contributions at various stages in the life of this ticket.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@7977 bcc190cf-cafb-0310-a4f2-bffc1f526a37