3748 actorselection helpers

Adds a serialization format in the style of ActorPath#toSerializationFormat. Also added more direct access to the anchor and path parts of an actor selection.

ticket is here: https://www.assembla.com/spaces/akka/simple_planner#/ticket:3748
discussion is here: https://groups.google.com/d/msg/akka-user/4whh2m6ttD4/dyty7EAnxXEJww

My personal use case was around stringifying actor selection so I'm not sure the API exposing the anchor & path are the best.

An alternative would be to move the protected pieces to a different name and allow the anchor & path to be the public API.

So, 
```scala 
protected[akka] val anchor:ActorRef
protected val path: immutable.IndexedSeq[SelectionPathElement]
```
becomes

```scala
protected[akka] val _anchor:ActorRef
protected val _path: immutable.IndexedSeq[SelectionPathElement]

val anchor:ActorPath = _anchor.path
val path: String = _path.mkString("/","/","")
```