Responsive Goodness

I started implementing a responsive layout for the Rails guides. I
haven't tested it in browsers yet beyond Webkit, but I wanted to get
feedback before putting more than a few hours into it. You can check
out my changes at:

Feel free to pull my fork and try it out on an iPhone. I'm planning on

1) expand/collapse for the chapters within a guide
2) in between state for tablet devices

Is this something that we want to continue on?