Minor bugfixing of the staticfiles app following upstream development in django-staticfiles.

- Create the files to ignore during the tests dynamically (.hidden and backup~)
- Refactored the post_processing method of the CachedFilesMixin storage mixin to be less time consuming.
- Refactored handling of fragments in the post_process method.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@17519 bcc190cf-cafb-0310-a4f2-bffc1f526a37