Adds Router.RemoveRouter

Fixes #439.

- Add `Router.RemoveRoutee()` and implicit casting an `ActorRef` to `ActorRefRoutee`.
- Add tests for `Router.AddRoutee` and simplify `AddRoutee` (that has the same logic copy-pasted in three functions).
- __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.