Update tab panel with new tab panel style

This makes use of the newer tab panel class, instead of the old/deprecated one. Also makes it so that the scrollbar is only visible if there's a need to scroll.

reviewer: @mvrable

![screen shot 2014-05-13 at 1 27 55 pm](https://cloud.githubusercontent.com/assets/4962198/2963629/7c3856ca-dadd-11e3-89a2-5048c04403ca.png)
![screen shot 2014-05-13 at 1 28 27 pm](https://cloud.githubusercontent.com/assets/4962198/2963630/7fdd4dee-dadd-11e3-9868-4679bb5ff644.png)