notify subscribers (private helper)
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.");
	}
}