Store eway transaction number back to the ec_reciept table

Pretty simple really, just added a line to ec_eway.module at around line 176

switch ($response->ewayTrxnStatus) {
      case 'True':
        $receipt->status = RECEIPT_STATUS_COMPLETED;
        $receipt->approval_code = $response->ewayAuthCode;
        $receipt->extern_id = $response->ewayTrxnNumber; //INSERTED THIS
        break;

      case 'False':
    

This just gives us a reference to the eway transaction number for reporting purposes