New class NzRayPicking

New class to pick a node using the ray method.

The names of the methods should be changed !

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.