Fixing ActorPath impl, reducing memory footprint.

Fix for #242 
This PR reimplements the internals of ActorPath, we no longer keep a full copy of each path element per node.

All ok?
This and all the similar changes (NoSender, StandardOutLogger) are actually correct as they were, so you need to revert them.

Scala code:
```scala
override val path: RootActorPath = new RootActorPath(Address("akka", "all-systems"), "/Nobody")
```
https://github.com/akka/akka/blob/master/akka-actor/src/main/scala/akka/actor/ActorRef.scala#L275 

These actors are Root Actors, meaning they do not have any supervisors, hence a `RootActorPath`.