F2.Events::on, ::many, ::once should return a cancellable event handle

(Enhancement request) Currently, on(), many(), once() return nothing and it's quite a pain to call F2.Events.off.

These methods should return an event handle with an off() method to make it easy to cancel a single event. F2.Events::off() can still be used to remove a large class of events.

I'll try to put up a jsfiddle next week.
