Integrate Grunt tooling for responsive images

[addyosmani]

I've added grunt-responsive-images to our tooling setup for this project with some basic configuration in place.

https://github.com/BBC-News/Imager.js/blob/master/Gruntfile.js#L28

It might be useful for us to drop in some hi-res sample images into app/images which we can hook up to this task. That way it can generate us the images needed for demonstrating how Imager.js works without the placeholder site.

We might need to figure out the best naming strategy for files however and how they'll hook into https://github.com/BBC-News/Imager.js/blob/master/app/scripts/imager.js#L63 and https://github.com/BBC-News/Imager.js/blob/master/app/scripts/imager.js#L115

[Integralist]

@addyosmani I believe this issue has been resolved so I'll close it. If not then just let me know.