[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)
just say `Color` instead. `Color` can actually accept a `str`, `tuple`, `list`, `ndarray`, etc., so it's easier just to say "Color" here so people refer to that. In fact you could do:
```
The `Color` to use when drawing the cube faces.
```
And then Sphinx should actually know how to link that to the `Color` instance. But maybe for now just fix the `tuple or Color` thing, since the linking should probably be done in *all* functions that use colors (not just this one).