ScalaBusMod improvements

- Added a type for the receive method (called Receive)
- NoReply object to not send a reply at all
- Be able to reply to a reply (Receiver and ReceiverWithTimeout)
- Added tests for missing/invalid actions

Especially the reply to messages is cool: It enables a user of this trait to set reply handlers to communicate directly with a client. The reply handlers can even have timeouts. See the `TestBusMod` class for basic usage. For better readability one should refactor the reply handler and timeout handler into their own `defs`.