public static function fetch($email, $size = 50)
 {
     $url = 'https://secure.gravatar.com/avatar/';
     $hash = md5($email);
     $args = array('d' => User::defaultAvatar(), 's' => $size, 'r' => 'pg');
     return $url . $hash . '?' . http_build_query($args);
 }