Handle init & external changes for onChange callbacks

Hello,

The basic idea comes from ON-OFF with a state-dependant style : 
I wanted the off to be red & the on to be green.

I managed to do that with onChange callback but 2 problems :
* this doesn't work for the initial state.
* this doesn't work on external model update.

[Check the Plunker](http://plnkr.co/edit/Hrh8WCxiCqdACeziLE2V?p=preview)

So here is a solution.