Improve the javadocs for the newly added HippoEventBus and add a default HippoEventListener interface such that developers only need to implement that interface

Currently, a developer has to do something like:

HippoEventBus eventBus = HippoServiceRegistry.getService(HippoEventBus.class);
eventBus.register(new Object() {

@Subscribe
void onEvent(HippoEvent event)

});

I think it would be nice, since the current usage is really geared for HippoEvent, that a developer can also do:

HippoEventBus eventBus = HippoServiceRegistry.getService(HippoEventBus.class);
eventBus.register(new HippoEventListener() {
@Override
void onEvent(HippoEvent event) {

}
});

Thus, the HippoEventListener interface already contains the 'subscribe' annotation, and also the correct type, namely HippoEvent

Also, the HippoEventBus can refer to this available listener interface : This will make its usage easier for developers, while keeping the same flexibility