Update gruntfile to v0.4

Hey! Thanks for this awesome lib! I'm working on integrating into [voxel.js](http://voxeljs.com) and thought I would contribute back.

I also work on [grunt](http://gruntjs.com) and saw you were using grunt. So though I would send a PR updating to the latest Grunt v0.4. Upgrading you would need to:

Uninstall grunt v0.3 and install the grunt-cli:
```
npm uninstall grunt -g && npm install grunt-cli -g
```

Then install the local dev dependencies with `npm install`.

Thanks again for the lib and the consideration!