[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)
I'll look into the rotation in a minute.

You should be able to pass `vertices` and `faces`, which IIRC are the outputs of the `create_cube` function (or close to it):

https://github.com/vispy/vispy/blob/master/vispy/scene/visuals/mesh.py#L74