New class NzRayPicking

- GetEverySceneNodeFromMousePosition(): Gets every nodes below the mouse
position.
- GetMousePositionFrom3DCoordinates(): Projects a 3D position to the
screen.
- GetRayFromMousePosition(): Returns the ray emitted by camera in the
direction of the mouse position.
- GetSceneNodeFromMousePosition(): Returns the first hit node who is
below the mouse position.