Add comments to new code (mostly KDTree-related).
Fix a memory leak in Mesh: we didn't delete the KDTree when done.
Fix valgrind warnings.

- always initialize the bumpmap to NULL in Node.
- generate dNdx / dNdy for Plane, Sphere and Cube.
Beautification: No need to use pointers to KDTreeNode in build()/intersectKD().

References are just fine, we don't do anything that really requires pointers.
Add new source files to .cbp projects.