Design improvements

Hey Team - I had a blast teaching this past weekend at my first RailsBridge! Thanks for your hard work :)

Based on student feedback, I decided to clean up the guides' CSS by simplifying the design, splitting the styles into separate files (no longer in erector), and by providing a consistent navigation sidebar.

Before:
![Screen shot 2013-01-24 at 3 29 12 PM](https://f.cloud.github.com/assets/105412/95776/e99d84e0-667d-11e2-9984-0e1927482c2c.png)

After:
![Screen shot 2013-01-24 at 3 27 57 PM](https://f.cloud.github.com/assets/105412/95773/b61a0a62-667d-11e2-83dc-39bdd9ee2849.png)

Comments welcomed, I'm happy to continue tweaking. A huge thanks to Carbon Five for sponsoring this work as one of our internal Project Days!