MessageTarget introduction

The MessageTarget interface is a small interface implemented by methods that can receive messages. Two examples of message targets are `ConsoleCommandSenderMock` and `EntityMock`.

Using MessageTarget

Any message that was sent to the target can be read using:

SimpleEntityMock entity = new SimpleEntityMock();
entity.sendMessage("Hello world!");
String message = entity.nextMessage();

It also contains two assert methods to check if a message was or wasn’t received.

entity.sendMessage("Hello world!");
entity.assertSaid("Hello world!");
entity.assertNoMoreSaid();