Added PagedLOD feature, with a new quadtree example. 

Also added computation of the projected bounding sphere size in pixels to the Lod class.
for vec3 use Vec3.create() instead of [0,0,0]. The chrome vm for example will first optimize it to an array of int but after will be recompiled to an array of float. So to avoid this, it's better to use Vec3.create() , it's also valid for all vector and matrix.