From BroadcastActorRef, remove the inheritance to ActorRef.

BroadcastActorRef isn't really an ActorRef. For example, it does not have path.
Maybe it should be call BroadcastActorCollection or something in line with that.
Make ActorRef.Path {get;} abstract instead of virtual

This is done in order to avoid calling virtual members from constructor
Add Dispatchers.DefaultGlobalDispatcher
Add InvalidMessageException
Add RootGuardianSupervisor, aka theOneWhoWalksTheBubblesOfSpaceTime & RootGuardianActorRef
Add EmptyLocalActorRef and let DeadLetterActorRef inherit it.

We need DeadLetterActorRef to be an InternalActorRef
Add Logger.GetLogger(Loggingbus) and handle string log sources
Refactored VirtualPathContainer.GetChild()

Switched to enumerator to avoid multiple enumerations
Add helper method to test if type implements other type.

Basically it just calls IsAssignableFrom, but I find it hard
hard to remember in which order the arguments should be.
Add constant Mailbox.Debug and mailbox tracing

The Debug constant is a compile time constant that can be set to true
before compiling in order to get mailbox tracing on standard out.
Change the order ActorRefs, Cells and Actors are created

- The ActorRef creates the Cell.
- The actor instance is created async, meaning we must make sure the mailbox
  isn't started and manually start it.
- Implemented RepointableActorRef (The tests for the router implicitly tests
  this)
- abstract class ActorRefProvider has become an interface (Akka's
  trait has no implementation, so it works just like an interface).
- ActorCell and ActorSystem are both implementing IActorRefFactory.
  In Akka ActorRefFactory is a trait with implementations for
  ActorSelection(). ActorRefFactoryShared has been added that contains
  these implementations.
- Fixed bug: ActorPath.Parse must respect casing for system name

All changes are in line with how Akka does it.
Merge pull request #162 from HCanber/ref-cell-actor-creation

ActorRefs creates Cells and create actor instances asynchronously