inc/class-rewrite.php - Resolve issue #304

While creating rewrite rules, preppend category slug by it's
ancestors slugs only if $taxonomy['category']->rewrite['hierarchical']
is true. Until now, users who set it to false got 404 error code
instead of subcategory pages.
Merge pull request #305 from Minio/master

inc/class-rewrite.php - Resolve issue #304