Voronoi sphere

I'll add a `voronoi3d` module to [dotSCAD](https://github.com/JustinSDK/dotSCAD) for creating a 3D version of the [Voronoi diagram](https://en.wikipedia.org/wiki/Voronoi_diagram). Here is a demonstration about how to use it.

The preview or rendering of 3D Voronoi is slow so the source code doesn't create the Voronoi sphere directly. You have to download the scad file, use [OpenSCAD](http://www.openscad.org/) to render and export the 3D Voronoi model first.

Then, update the `stl_file` in t