[WIP] Frontend cache invalidation

This pull request adds support for automatically purging pages from a frontend cache (varnish, squid, etc) when a page is updated or deleted. It also provides a utils function to allow purging of pages in other situations (such as purging a listing page when an item is added/changed/deleted).
Do we need to reference Varnish here? I believe Squid handles PURGE requests in just the same way:

http://wiki.squid-cache.org/SquidFaq/SquidLogs#Request_methods

`cache_server_url` might be better.