Add toString method to ProtocolNode class

This change is very helpful when using the events and want to print this object or convert it to JSON format.

For example:

```php
    /**
     * Event fired when was get an error
     *
     * @param string $phone
     * @param \WhatsApi\Nodes\ProtocolNode $error
     */
    public function onGetError(
      $phone,
      $error
    )
    {
      $result = array(
        'sender' => $phone,
        'error'  => (string)$error  //Casting to force object conversion to string
      );

      echo json_encode( $result );
    }