code: function(subscribers, value) {
  	if(!subscribers) return;
  	
  	var subs = subscribers.slice();
  	for(var i = 0; i < subs.length; i++) {
  		if(typeof subs[i] == "function") subs[i](value);
  		else if(typeof subs[i].call == "function") subs[i].call(value);
  		else throw new TypeError("Observer is not callable.");
  	}
  }
docs: notify subscribers (private helper)