Enable materialFlatColor option

![screen shot 2013-06-03 at 11 25 06 pm](https://f.cloud.github.com/assets/99604/603925/1de6db56-cce1-11e2-8c1a-e2934b10c5d4.png)

voxel-texture@0.5.6 now has a `materialFlatColor` option. This will disable textures and use `new this.THREE.MeshBasicMaterial({vertexColors: this.THREE.VertexColors})`.

You can specify colors in the materials array:

```js
var game = createGame({
  //materials: ['brick', ['grass', 'dirt', 'grass_dirt'], 'dirt'],
  materials: ['#E04006', '#C0D890', '#8A360F'],
  materialFlatColor: true
})
```

The AO isn't that great but I'll make it better in time. Thanks!