Lights are now intersectable.

A Light must implement an ::intersect() method. raytrace() checks
for such intersections and returns the light color in this case.
Speed up K-d tree intersection.

Add BBox::intersectWall() method, which is used in intersectKD().
The speedup is as much as 30% (see http://raytracing-bg.net/?q=node/68
for thorough explanation).