pingback library requirements

Client
======
- send pingback for given URL to remote URL

 $pc = new Pingback_Client();
 $pc->send($myUrl, $targetUrl);

TODO: error handling. exceptions? simply true/false with $pc->getError()?

Server
======
- register custom handler
- register custom validator

 $ps = new Pingback_Server();
 $ps->registerHandler(array($datastore, 'storePingback'));
 $ps->registerValidator(array($pingbackValidator, 'checkBlogAge'));
 $ps->registerValidator(array($pingbackValidator, 'checkFriendliness'));
 $ps->run();