Added "grunt-cli" to devDependencies

After cloning bilby and installing nodejs, I ran into this error:
```
$ npm test

> bilby@0.1.0 test /vagrant/Developer/bilby.js
> grunt rig jshint nodeunit uglify emu

sh: grunt: not found
```
I think the reason is this: "Grunt no longer ships with a binary. In order to get the grunt command, install grunt-cli."

http://gruntjs.com/blog/2013-02-18-grunt-0.4.0-released

So in this pull request, I propose to add grunt-cli to the devDependencies.

Cheers!
Sébastien.