Add class on label

Hi, I have made a few change in order to customize label with css class. 
I have tested it with single and multiple selection, but my be there is some side effects that I am not aware of.

Example :
![capture_sumo](https://cloud.githubusercontent.com/assets/7304827/4872112/f756defc-61d9-11e4-8011-1b620d85912b.PNG)
![capture_sumo_2](https://cloud.githubusercontent.com/assets/7304827/4872114/1f781cac-61da-11e4-83b4-3d5667f73231.PNG)