Description: Sets or gets the callback function to be invoked before dismissing the notification [ if the invocation result === false , notification dismissal will be canceled ].
Default:undefined
var canDismiss = false;
var notification = alertify.success('You can dismiss this after 5 seconds!');
notification.ondismiss = function(){return canDismiss; };
setTimeout(function(){ canDismiss = true;}, 5000);