Inheritence for Services resources

As we use class RESTServer for our REST server I think it will be very good feature to let users use inheritance here.

Practically what I mean is to add new attribute to hook_server_info named "class".

In this case in services_endpoint_callback() instead of calling

print call_user_func($server . '_server');

We would load server class and run:

  try {
    $server = new $server_class;
    return $server->handle($canonical_path, $endpoint_path);
  }
  catch (Exception $e) {
    $server->handleException($e);
  }

And if developer will need to change some behavior inside the class he can easily change the class of server using hook_server_info_alter and inherit his own class from RESTServer.