Adds enqueueEvent method to World::Object.

This method will add events to the Object that should be handled during
the frame.

I should only add events that exist within the pool of events the Object
responds to.
Adds methods to World::Object to manage active events.

respondTo will add the given event as one that this object responds to.

ignore will remove the given event as one that this object responds to.

respondsTo will query whether or not the given event is one that the
object responds to.
Adds dequeueEvent to World::Object.

This method will pull the next event for the object to respond.
Adds hasEvents() to World::Object.

This method will poll whether or not there are unhandled events on the
event queue. Returns true if so.
Adds documentation for _responds_to member of World::Object.
Updates World::Object::respondsTo() to be const.

It queries... it doesn't need to modify the class. Make sure the
compiler knows this.
Fixes World::Object::dequeueEvent() to actually return the event.