[WIP] 3D Visuals

This pull request implements basic 3D visuals, like a 3D polyhedron or a 3D cube. I think it is a prerequisite for completing volume rendering as a visual. (https://github.com/vispy/vispy/pull/458)
1) How should one pass a list of vertex indices to the Mesh visual? That wasn't clear to me.
2) Any thoughts on the rotation failure?