Send Terminated when trying to watch already terminated actors
Merge pull request #192 from HCanber/terminated-watching-dead

Send Terminated when trying to watch already terminated actors.

When an actor terminates, it swaps the mailbox to DeadLetterMailbox (new class).

When the terminated actor receives the Watch message DeadLetterMailbox will forward it as a DeadLetter to DeadLetterActorRef. It inspects the message inside the DeadLetter, sees it is a Watch and sends watcher a DeathWatchNotification message, which will cause the watcher to send itself a Terminated message.