Fixed #5893 -- Added a flag to FilePathField to allow listing folders, in addition to regular files.  Thank you to Brian Rosner, for encouraging me to first contribute to Django 4 years ago.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@17925 bcc190cf-cafb-0310-a4f2-bffc1f526a37