Add tests for Router.AddRoutee
Add Router.RemoveRoutee & implicit casting ActorRef -> ActorRefRoutee

Simplify AddRoutee, that has the same logic copy-pasted.

Breaking change: Router.Send should not be public. This can never have been meant to be public and I can't see why anyone would actually use it from the outside. The public send-to-routees is Route(). The impact of this breaking change should be slim to nothing.
Clearer tests
Merge pull request #441 from HCanber/router-439

Adds Router.RemoveRouter