Add IsNull and HasValue methods to Dynamic* classes

**Breaking Change: DynamicNull.HasValue and DynamicNull.IsNull have been changed from properties to methods
Rename umbraco.MacroEngines.ExtensionMethods as umbraco.MacroEngines.PrivateExtensionMethods
Implement extension method invokes for DynamicNode

umbraco.MacroEngines.Juno/RazorDynamicNode/DynamicNodeList.cs

@@ -235,5 +235,13 @@ namespace umbraco.MacroEngines
                 this.Items.Remove(node);
             }
         }
+        public bool IsNull()
+        {
+            return false;
+        }
+        public bool HasValue()
+        {
+            return true;
+        }
     }
 }