Add support for .once and .off

[TheRealest]

• .once: like .on but after the callback has been triggered by an event one time it is deleted.
• .off: takes an optional event name and callback -- if an event name and callback are supplied, remove the callback from listening to the given event; if only the event name is provided, remove all callbacks from that event; with no arguments removes all eventful callbacks for the given component.