Compute vertex colors for ambient occlusion.

Its not perfect but a little closer. If two voxels are stacked on each other it's a little off. There is probably a correct way to better compute ambient occlusion. This probably should become it's own module as well. With more enhancements I imagine it could become quite involved, i.e.: simulate shadowing on neighbor voxels, adjusting the light/shadow based on the type of voxel, etc.

![voxel](https://gist.github.com/shama/5210665/raw/eb0bf09c92fbe3fc0e9b01bbb40f98ae5ecf26b3/screen_2013-03-20.png)

`voxel-texture` has been updated to paint materials based on `face.color` rather than `face.vertexColors[0]` to support this: shama/voxel-texture@6043b494a359d59026edcaf307975547f56214ca and published as `voxel-texture@0.3.2`

I'll keep at it, fun stuff. Thanks!