issue #72, Exipration stuff

Plenty of questionable things in here: 
- The interface of `org.ehcache.expiry.Expiry`
- The way `Expiry` gets into a Cache and then onto the Store, as explicit methods on the config objects
- The way the `TimeSource` gets into OnHeapStore (no idea if I used the service config concept correctly)
  - see `org.ehcache.internal.store.OnHeapStore.Provider`
- updates to Store javadoc to mention expiration detail is in PR #130
I tried pondering that myself yesterday night, but I really have a hard time qualifying an exception being thrown there, and what should happen to the mapping. 
Here it'd be what should happen to the old mapping I guess, but in case of a get, and the `Expiry.getExpiryForAccess` throwing, what should happen there? Honestly... still not quite sure.