refact(es6): use es6 syntax.

* fixed #10, But iojs-2 not support Object.assign, so use object-assign
  shim instead for now.
* use es6 syntax.
* use eslint to instead jshint.

test/locales/zh-CN.js

@@ -1,3 +1,5 @@
+'use strict';
+
 module.exports = {
   Email: '邮箱',
   'Hello %s, how are you today?': '%s，今天过得如何？',
@@ -5,8 +7,8 @@ module.exports = {
     user: {
       fields: {
         name: '姓名',
-        gender: '性别'
-      }
-    }
-  }
+        gender: '性别',
+      },
+    },
+  },
 };