Condensed number of tree classes to 4 instead of 6
BUG FIX: Bounding Boxes were not getting created properly in cases where min=Vector([0,0,0]).  This was because of the __nonzero__ check on Vector.  I removed that since it was a bit confusing and updated occurrences of it with Vector.length().
Renamed cube shader to skin and transferred responsibility of shader binding to render functions.