Allow TraceablePDO to accept PDO subclasses

I'm using a subclass of PDO in my code and I had to subclass TraceablePDO in order to use it. I think this change would better fit in the original TraceablePDO.

    class TraceablePDO extends \DebugBar\DataCollector\PDO\TraceablePDO
    {
        public function __call($name, array $arguments)
        {
            return call_user_func_array(array($this->pdo, $name), $arguments);
        }
    }