Headers setting support when files match a regex

Sometimes it might be useful being able to assign to a file a specific header to be returned by RackSpace when serving the file to the clients.

E.g. it is useful for font files that sometimes need to include an `Access-Control-Allow-Origin: *` header in order to be accessible cross-domain.

This PR tries to add to posthaste a functionality similar to the one provide by Django-cumulus headers (http://django-cumulus.readthedocs.org/en/latest/#headers).