Fix crashes in event dispatch other related safety checks/fixes

1: Add a fix to prevent events from being sent to scene nodes which are destroyed. Previously when a node was being destroyed it would try to unregister any event listeners it would have. This process would fail however in the case where event dispatching was already underway because we can't modify the listeners list while we are iterating through it. This is a pretty common occurrence since we often tear down & switch scenes in response to button clicks etc. Fix the problem by nulling out the slot for the listener we are removing in this case, so that it's node no longer receives any events after it is destroyed. The event dispatching code will cleanup this unused/nulled slot once the event processing loop has terminated.

2: When removing an event listener in cocos2d::EventDispatcher, ensure immediately it gets removed from the node priority map and dirty nodes set in order to ensure we don't inadvertently access any dangling pointers to nodes.

3: Add debug checks to ensure nodes have no associated event listeners after they are destroyed. This check will catch the problem fixed by (1) if it is still present.

4: Add some extra debug sanity checks in the event dispatcher to ensure nodes are not added to the lists twice and that they are being properly removed from the lists under certain conditions.

5: In cocos2d::Node's destructor NULL out members after releasing them in case they somehow happen to be accessed during the destructor. Move the release of the event dispatcher to the very end, since we need to check against it in debug.