[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)
Okay, I'll change the docstring. As for vertex_colors and face_colors ... ? Stay or go? (I'd like them to stay.)