Subscribe for changes of a property or the whole object
function(property, callback) {
		if(typeof property !=  "string") {
			callback = property;
			property = "__all__";
		}
		
		this._subscribers[property] = this._subscribers[property] || [];
		
		if(this._subscribers[property].indexOf(callback) == -1) { 
			this._subscribers[property].push(callback);
		}
	}