Fixed #6201 -- Improved the {% cache %} template tag to allow the timeout to...

Fixed #6201 -- Improved the {% cache %} template tag to allow the timeout to be a template variable. Inspired by the patch by zz and edrik

git-svn-id: http://code.djangoproject.com/svn/django/trunk@7754 bcc190cf-cafb-0310-a4f2-bffc1f526a37