User interface

Navigation between group views now uses icons instead of labels.

Before:
![labels](https://cloud.githubusercontent.com/assets/6856120/3372510/a5a81394-fbab-11e3-844d-f72e7c2e605c.png)

After:
![icons](https://cloud.githubusercontent.com/assets/6856120/3372509/a5a2f68e-fbab-11e3-9994-1e3763043c8e.png)

Tooltip (unstyled):
![icons-tooltip](https://cloud.githubusercontent.com/assets/6856120/3372511/a5a90d4e-fbab-11e3-860e-62990a259fa6.png)