Support signed URLs

[Fred Carlsen]

$builder = new UrlBuilder("demos.imgix.net");
$builder->setSignKey("test1234");
$params = array("w" => 100, "h" => 100);
echo $builder->createURL("bridge.png", $params);

// Prints out:
// http://demos.imgix.net/bridge.png?h=100&w=100&s=bb8f3a2ab832e35997456823272103a4