public function addAttachment($name, $signer_index, $instructions = null, $required = null)
    {
        $attachment = new Attachment(array(
                'name' => $name,
                'instructions' => $instructions,
                'signer_index' => $signer_index,
                'required' => $required
            ));

        $this->attachments[] = $attachment;

        return $this;
    }