[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)
Will update the docstring if @lcampagn is okay with vertex_colors and face_colors in the args. The vertex_color, face_color, and color arg are passed to the underlying mesh, so should behave the same as for them.