Dropdown support for tabbars

Added dropdown support for tabbars, which means that you can specifiy an optional groupname per pane and all panes with the same groupname will be grouped together in a dropdown-as-a-tab.

Although I was able to test this by editing the compiled bootstrap-ui-0.1.0.js directly (and the templates), I was unable to verify the changes in the bootstrap-ui repo itself. For some reason, using cygwin, a clean cloned fork already yielded 121 failing tests.

![tabs_1](https://f.cloud.github.com/assets/2115718/164979/9163ee10-7909-11e2-98d6-6bdd02be01e2.png)
![tabs_2](https://f.cloud.github.com/assets/2115718/164980/93ad892e-7909-11e2-91c1-93062b9fa6d5.png)