Added another subscription method to wicket-atmosphere.

Wicket-atmosphere provides only the method-subscription method. The nice concept of the subscription methods have one penalty: It is not possible to add a filter with context variables before the request context is loaded. E.g. self-made channels or other checks with variables of components.

I hope there is no error in concept. 

Example:

	@Override
	protected void onInitialize() {
		super.onInitialize();
		
		final Number id = getWatchId();
		
		AtmosphereEventSubscriber s = new AtmosphereEventSubscriber(this);
		s.subscribe(CommentEvent.class, new AtmosphereEventSubscription<CommentEvent>() {
			@Override
			public void call(AjaxRequestTarget target, CommentEvent event) {
				logger.debug("Comment with id=" + event.getId());
			}
		}, new Predicate<CommentEvent>() {
			@Override
			public boolean apply(CommentEvent event) {
				return id.equals(event.getId());
			}
		});
	}