taxonomy class fix

Please let me know if I'm wrong, but I don't see any reason for attributes or shipping classes to be hierarchical. This change would allows us to create a general query to get "Categories" with:
 ```` PHP
$taxonomies = get_object_taxonomies($post_type->name, 'objects');
  foreach($taxonomies as $tax) {
    if($tax->hierarchical == true) {
      // 'Product Categories'}
````