Added responsive stylesheet

It's a start. Should look better on Mobile and iPads. If it's not working on the actual device (I only checked on my laptop browser), I think there is a meta tag you can add to the layout to make it work. Something like this, but I wasn't sure how to add it with Haml:

<meta name="viewport" content="width=device-width, initial-scale=1.0, maximum-scale=1.0">