Tagging cached pages paths

This is something I was building for myself but I thought I'd offer to merge it into master.  

I use a lot of full page caching and wanted a way to tag the paths I'm caching with cashier.  The implementation isn't as clean as the hooking into Rails.cache.write because there aren't easy ways to hook extra options into `caches_page`, this requires telling cashier directly about each page cache with the `cashier_pages` method.  If you see something I missed, please let me know.  

The clearing of page paths is handled by [Maidservice](https://github.com/scotterc/maidservice), a gem I wrote using redis' pub/sub to clear page caches on multiple servers.  It's worked well in production and is a good stand in for `expire_page`.

This isn't ready for general use because I haven't disabled it for cache_store use, but I wanted to get it out there in case there's interest.  

It also includes commits from https://github.com/ahawkins/cashier/pull/15 because I have been using that for the past year and it's necessary for evaluating procs.