For review comments only - issue #72, Exipration stuff

Plenty of questionable things in here: 
- The interface of `org.ehcache.expiry.Expiry`
- The implementation of expiration in OnHeapStore
  - in particular the while(true) loops. I think it could be reasonable to return without looping even though it is known a newer/more-accurate answer might be observable
- 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`
- no updated javadocs (in particular updating `Store` methods to talk about expiration
- no new tests
Yeah, that's it... but true that! Rather remove the limitation later than the other way around. Again, with 107 in mind, this passes all test! 

Now, I still think that point 1 is relevant... and if we convert to "smallest unit" supported (i.e. MILLISECONDS), it would always resolve... including to possibly 0. Anyways we can worry about this later. If you decide to leave it as so, could you just file an issue wrt that?