Adding a package.json to lower the barrier to start contributing

When I started playing around with Lungo.js directly, it kinda put me off that all the grunt-plugins etc. needed to be installed manually, as there wasn't a package.json.

So, well, I added one. Now getting ready to hack&build is (more or less) just

    $ npm install

If you have some adjustments to suggest, I'd be more than happy to incorporate those.