Support for pluggable service esponses

Would be excellent we could provide a method for extra response types that come out of the get_server module, such as vCard etc.

An implementation of this just looks like

function mymodule_services_encode_response($protocol, $result, $method) {
  switch ($protocol) {
    case 'vcard':
       header("Content-type: text/x-vcard");
       header("Content-Disposition: attachment; filename=vcard.vcf");      
       ...
       print $vcard;
       exit;
    break;
  }
}

See attached patch