Fix Memory Leak for events OnCameraVisibleBoundsChanged, SceneViewportChanged and LayerVisibleBoundsChanged.
+ Make sure events are un attached when removed from parent and cleanup = true is specified.
- The problem with the event handlers was that they were being created as "= delete {}" which means that there was no way to really un attach the event handlers once they were created.  So to solve this concrete event handler delegates were created which resolves this issue.

This should clear the issue https://github.com/mono/CocosSharp/issues/66