added sort for users list

app/views/admin/user/_table_list.html.erb

@@ -1,8 +1,8 @@
 <table cellspacing="0" cellpadding="0">
   <tr>
-    <th>User</th>
-    <th>Email</th>
-    <th>Role</th>
+    <th><%= sortable "name", "User" %></th>
+    <th><%= sortable "email" %></th>
+    <th><%= sortable "role" %></th>
     <th></th>
   </tr>