#18 Enable/Disable culling support

[Hernan Saez]

Implement a way to enable/disable culling support for a single Node object or an entire scene graph branch. That way, objects in the scene can be tagged as not to be culled during a culling a operation.

It should not add more complexity to the Node or Group classes. Instead, a RenderState object can be used.

Node: If using a RenderState, the setEnabled()/isEnabled() method pair that can be found in some of the RenderState implementations can be moved to the base class.