Underlined TabControl

- [x] Add the underline
- [ ] Figure out where we enable this behavior

Fixes #960 

![2014-06-14 14 33 23 mahapps metro - demo application](https://cloud.githubusercontent.com/assets/964691/3278456/8f0fa59a-f3c0-11e3-9c7c-19386820e58e.png)