code: 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);
  		}
  	}
docs: Subscribe for changes of a property or the whole object