Custom event & context

Hi.

I really like your plugin! But two aspects bothered me:

1. There was no option to set a context. So every search for the specified DOM-node happened inside the whole DOM-tree. That's why I added the `context`-option. If it's set, the `$.contains`-method looks inside the context (a smaller part of the DOM-tree).

2. There was no custom-event. As the plugin does some kind of event-listining, I thought there has to be a custom event which works with the `$.fn.on`-method like every other event does in the jQuery-library. So I added this event, which internally uses the `$.fn.whenLive`-method.

Let me know what you think of my modifications.

Best regards
Emanuel