* Replaced the RDF support (rdf.php) with a newer version (backend.php).
  The new version is a generic framework that has everything ready to add
  support RSS and XML backends in a 100% transparant way.  It's a flexible
  framework. Other changes include: better coding, improved robustness and
  readability.
* RSS and XML support will be integrated in near future.
* Cache-invalidation is set to 30 minutes.