Improve usability of sheet tabs

This attempts to improve on the current tab implementation by adding a border to the bottom of each of the tabs. This is then red when active. This should hopefully remove the confusion with the active text looking like a link.

![screen shot 2014-07-24 at 15 07 11](https://cloud.githubusercontent.com/assets/47144/3688209/718cc6f0-1333-11e4-9dc5-831d6d964ffa.png)
Cheers.