Added `odometer-digit-added` event

I needed a way to set the odometer's width in order to smoothly animate it when the number of digits increased/decreased, so I ended up adding an `odometer-digit-added` event which is triggered every time a digit is inserted to the DOM.

You can check the [JSFiddle I created](http://jsfiddle.net/Q6ASx/13/) in order to showcase the end result (I included the compiled JS result code in the commit only to be able to use it from the JSFiddle as an external resource).

There might probably be a better way to handle this, but I don't know the source code well enough to determine that.