simplified GLIcoSphere's vertex normal computation

it's a sphere - no need to mess with adjacent faces. all normals are in the
same direction as the vertex itself!
assets folder re-included
Formatting for GLCubeSphere

Biggest change is that GLCubeSphere is now a subclass of GLMesh.
Other than that, there is a lot of commenting, formatting, and
consistency-making.

known bug(?) using assets/mars as the cubesphere texture shows up
as a big purple planet.