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.
Right, something felt wrong about this. I thought adding the forward to EventBus clearly spelled out the EB had the forwarding capability. Forwarding is really just a send I agree with that, but it's different in that the message already exists and is handled differently in that message isn't actually being created. 

I am not married to either approach so I am more than willing to taking it out of the public API since no one is using it yet.