Add support for GruntJS

[GruntJS](http://gruntjs.com) is a great tool for running tasks.

One time setup per computer:
`sudo npm install -g grunt-cli`

One time setup for project:
`npm install`

Then whenever you're working on Holder and you want to create a minified `holder.min.js` then just run:
`grunt`

That's it! This opens the door for a lot of cool stuff. JS linting, unit tests, continuos integration and merge-checking.