Add switchClass binding

This adds a new binding called `switchClass`. It's pretty much the `switch` binding, except instead of hiding and showing elements, it adds a class to an element when the property's value matches the case. My use case for this is setting a button to an active state based on a `mode` property on the model in my view. 

Hopefully that makes sense! :laughing: Let me know if I can make it clearer. 


![selfie-1](http://i.imgur.com/cL4XKt5.gif)