support varnish management port

[dbu]

instead of sending special requests to the varnish frontend (and have rules that check every request for being a purge from the webserver), one could also use the telnet interface to send purge requests.

the user of the bundle should be able to call the same methods whether he uses the frontend purge controller or the one using the administration port.

http://www.varnish-cache.org/docs/3.0/reference/varnishd.html#management-interface
http://www.varnish-cache.org/docs/2.1/reference/varnishd.html#management-interface

see also TODO in https://github.com/liip/CacheControlBundle/blob/master/Helper/Varnish.php

[dbu]

belongs into the cache proxy client, not into the bundle: FriendsOfSymfony/FOSHttpCache#61