Add getBoundingBox() to Node and an example that autoplace a ground to the scene

The ground is correctly set at the lowest height of the scene.
Works great on some models, ogre, bob and shark
Do not work correctly on model which have a global/root/master matrix transform in the scene graph
Tried to transform the bounding box with this same matrix but results are still slightly incorrect.
Doing this also imply we know which matrix to use from the graph and if it needs to be transposed.