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.

akka-docs/java/code/akka/docs/extension/ExtensionDocTestBase.java

@@ -42,7 +42,7 @@ public class ExtensionDocTestBase {
 
     //This method will be called by Akka
     // to instantiate our Extension
-    public CountExtensionImpl createExtension(ActorSystemImpl system) {
+    public CountExtensionImpl createExtension(ExtendedActorSystem system) {
       return new CountExtensionImpl();
     }
   }