refactor Use and Defs element with native code support(iOS)

lib/attributes.js

@@ -3,7 +3,7 @@ import _ from 'lodash';
 const merge = _.assign;
 
 function arrayDiffer(a, b) {
-    if (a == null || b == null) {
+    if (_.isNil(a) || _.isNil(b) ) {
         return true;
     }
     if (a.length !== b.length) {
@@ -79,6 +79,10 @@ const GroupAttributes = merge(NodeAttributes, {
     clipRule: true
 });
 
+const UseAttributes = merge(RenderableAttributes, {
+    href: true
+});
+
 const PathAttributes = merge(RenderableAttributes, {
     d: {
         diff: arrayDiffer
@@ -146,5 +150,6 @@ export {
     EllipseAttributes,
     ImageAttributes,
     LineAttributes,
-    RectAttributes
+    RectAttributes,
+    UseAttributes
 };