Add "delete" event

[eschwartz]

I need to know when a cache entry is deleted, so I can do some additional cleanup on it.

For example, let's say an entry is an object that contributes to some global state over time. Even though my object is deleted from cache, the global state will still remain in memory. If I have a delete event hook, I can tell my entry to clean up it's global state.

I'm working on it :)