Fixed #5833 -- Modified the admin list filters to be easier to customize. Many thanks to Honza Král, Tom X. Tobin, gerdemb, eandre, sciyoshi, bendavis78 and Julien Phalip for working on this.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16144 bcc190cf-cafb-0310-a4f2-bffc1f526a37