Added AllValuesFilterSpec to admin changelist filters, which lets you put any...

Added AllValuesFilterSpec to admin changelist filters, which lets you put any arbitrary field in Admin.list_filter. To determine the list of all available choices, Django does a SELECT DISTINCT. Note this is backwards-incompatible for people who have defined and registered their own FilterSpecs, because each FilterSpec now takes a 'model' parameter.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@3136 bcc190cf-cafb-0310-a4f2-bffc1f526a37