Add data-labels attribute to turn knob into a multiple choice switch

add `data-labels` attribute which accepts a comma delim string as a list of labels to be displayed on the dial. Good in the use case of making the knob into a multi-valued switch, which could be used as an option button in form or similar. Searched for a while but haven't seen this feature in any other libraries

P/S: there seems to be a lot of changed but actually it's caused by prettify plugin from sublime. 
![knob_with_labels](https://cloud.githubusercontent.com/assets/537655/2888883/3647427c-d513-11e3-9998-0fbf2d902383.png)