Info button in interactionbtngroup example makes an error

There are several problems:
* the ng-show value of the button is incorrect
* no control is linked to the button => buttonsCtrl is null in the directive but goog.isDef(null) returns true => buttonsCtrl is called whereas is null.

I have added a ng-cloak directive to the "info" div to prevent it from flashing when the page loads.
I did not realize that the "info" button displayed an alert div. Thanks for catching this.