jboner/akka — pull request #251 by rkuhn (2012-01-24 10:35:54+00:00)
Title: add ExtendedActorSystem to shield ActorSystemImpl

- add provider, guardian, systemGuardian and deathWatch to it
- make ActorSystemImpl extend ExtendedActorSystem
- use ExtendedActorSystem for creating extensions, thereby limiting the
access extensions get to just those four published methods.