Event handlers with callback de-registration

Have event handlers return a callback registration.
MVP testing code

Eventbus-driven GWT-style MVP coding. MVP - Model, View, Presenter.
Uses Activities, Views and Places, you need to add <inherits
name="com.google.gwt.activity.Activity"/>  to you *.gwt.xml file (and
insure that this Titanium4J.jar is higher than your GWT code in your
classpath - check Eclipse Java Build path and other T4J tutorials)
Merge pull request #1 from mvniekerk/master

MVP and event callback registration