Use __dirname in glsl paths?

https://github.com/stackgl/glslify#browserify-transform uses `__dirname +` for shader filename paths:

``` javascript
var src = glslify(__dirname + '/shader.glsl')
```

voxel-shader probably should to, instead of `./` relative:

``` javascript
    glslify('./lib/ao.vsh'),
    glslify('./lib/ao.fsh')
```
