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.
The issue was, none of the existing methods on the EventBus were appropriate to handle the forward as the send/publish methods create a new message prior to send or publish. Obviously we don't need this for forward so I thought creating a specific forward method was appropriate.