allow extensible event delivery in wicket-atmosphere

[dretzlaff]

This commit allows applications to register a custom subscription collector, and a custom subscription invoker, with the EventBus. The Guava-like, @Subscribe-annotated behavior is retained as default. Our application uses listener interfaces for notifications, and this change lets us use these interfaces to notify Wicket components instead of trying to switch our pub/sub syntax just for the UI.

[papegaaij]

Thanks for the patch!. I like the idea. However, EventSubscriptionInvoker could use a bit more documentation. Also, the return value of invoke does not seem to be used. I think it should either be void or it should be used. If you could address these minor issues, I'm ok with merging this.

[dretzlaff]

Good eye on the return type. That was cruft from before AjaxRequestInitializer was introduced. I also improved the invoker javadoc. It didn't feel appropriate to describe our app's implementation, but FYI our listener interfaces can contain multiple parameters. We needed a separate invoker impl to extract them from the event payload into separate invoke args.

[papegaaij]

It seems Github does not automatically close pull requests when they are merged. Can you close this?