Guides sidebar position fixed on scroll.

Been using [Padrino guides](http://www.padrinorb.com/guides) a lot lately and found myself scrolling up and down quite a bit.

Thought it would be more convenient to have that sidebar fixed when scrolling. It stops just before the comments.

I didn't know how you wanted your js, so I simply wrote it in guides/show.haml. (hey, http request free!).
One thing though, it requires the sidebar to be wraped into a container, so I also edited guides/_sidebar.haml.

Performance wise I wouldn't worry, css is doing most of the illusion and there's no selector in the scroll callback (like most of the jQuery plugins you could find).