Allow for custom block materials

Based on the changes made in shama/voxel-texture#4, this enables two additional parameters for the game constructor: `materialType` and `materialParams`, which allow you to specify the default materials to use and alter their behaviour.

This means it *could* be pretty straightforward dropping in shaders for day/night transitions, warping terrain, etc. Unfortunately this only applies to the voxels at the moment (see below).

It'd be interesting to have a more flexible "base" material than `MeshLambertMaterial` which other modules could use or extend. Maybe there's a better way to go about it though, I'm not sure :)

![Screen Shot 2013-01-23 at 12 00 44 AM](https://f.cloud.github.com/assets/569817/86321/ce108f40-6495-11e2-8c48-e4e1085f7aa8.png)