Adding message forward capability to vertx-core

Simple functionality for message forwarding. Note, this will probably require a bit more work, but for now the basic functionality is in place. See MessageTest unit test for how it works.
A simple status flag so that a consumer or the message can see if it's been sent as a result of a forward, rather than a send/pub. I thought it might be useful for developers that may need to understand how the message was received.