feat(ngFlip): Added ngFlip directive.

Implementation of a flip switch with syntax:

     <div jqm-flip ng-model="flip" on-label="On" on-value="1" off-label="Off" off-value="0">
       My value is: {{flip}}
     </div>

This directive also plays nicely with jqm-theme and jqm-controlgroup.
This should resolve #12