Fix extend by using util.inherits

lib/model-builder.js

@@ -135,9 +135,9 @@ ModelBuilder.prototype.define = function defineClass(className, properties, sett
     for (var i in ModelBaseClass) {
         ModelClass[i] = ModelBaseClass[i];
     }
-    for (var j in ModelBaseClass.prototype) {
-        ModelClass.prototype[j] = ModelBaseClass.prototype[j];
-    }
+    
+    util.inherits(ModelClass, ModelBaseClass);
+    
     ModelClass.getter = {};
     ModelClass.setter = {};
     standartize(properties, settings);