defined a read only property 'data' to avoid breaking the api

lib/DataSet.js

@@ -274,7 +274,15 @@ DataSet.prototype.update = function (data, senderId) {
     this._trigger('add', {items: addedIds}, senderId);
   }
   if (updatedIds.length) {
-    this._trigger('update', {items: updatedIds, data: updatedData}, senderId);
+	var props = { items: updatedIds, oldData: updatedData };
+	// TODO: remove deprecated property 'data' some day
+	Object.defineProperty(props, 'data', {
+	  'get': (function() {
+	    console.warn('Property data is deprecated. Use DataSet.get(ids) to retrieve the new data, use the oldData property on this object to get the old data');
+	    return this.get(updatedIds);
+	  }).bind(this)
+	});
+    this._trigger('update', props, senderId);
   }
 
   return addedIds.concat(updatedIds);