[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)
Thanks for the docstring! I added it and the edge_color argument.

I actually think we should keep the vertex_color and face_color arguments, they can be handy. I want to use the vertex_color argument, although I could do it with a mesh now. And people may want a Cube with different colored faces.