Implicitly cast a TestProbe to an ActorRef

This PR makes `TestProbe` implicitly castable to `ActorRef`.

Tests requiring the `ActorRef` of a `TestProbe` can now be simplified:
``` C#
var probe = CreateTestProbe();
var sut = ActorOf<GreeterActor>();
sut.Tell("Akka", probe); // previously probe.Ref was required
probe.ExpectMsg("Hi Akka!");
```