Adding support for optional MB payment maximum date

When a maxdate is set during the creation of a Payment Identifier, easypay will not accept payments after the date expiration. 
[Link to documentation] (https://docs.easypay.pt/?api=requestPaymentIdentifier)


Actually, `self::MAX_DATE_FORMAT` would be the right keyword to use, as it prevents subclasses to redefined `MAX_DATE_FORMAT`