Add a 5-second tutorial to the top of the site, and reorganize some of the main info.

This will finally provide an authoritative source for the most trivial rack use cases.

I also changed the site to a basic two-column layout, with the incredibly-long News section on the left and everything else on the right. This makes it much easier to scroll down and see what information is available, as opposed to getting lost in the News and Releases lists.

You can see what it looks like [here](http://htmlpreview.github.io/?https://github.com/mcantor/rack.github.com/blob/master/index.html).

The CSS has been tested in Chrome and Safari.

Syntax Highlighting is achieved with a CDNJS copy of the "syntaxhighlighter" library. I'm not a fan of the color scheme, but couldn't find a better theme with a non-white background (white backgrounds just didn't stand out enough).