Add new XmlStatusResponse with another one "isSuccessful" method

[Axent96]

If we ask transaction status with XmlStatusRequest, we get a general XmlResponse. The method "isSuccessful" of the class is:
return $this->getErrorCode() === null;
If we ask some declined transaction with some errorCode then we get false for method "isSuccessful". But in the case we wont to know the status independent of transaction status. We can have here for example a declined status too & with this status the response must be successful but this is a response and not a valid transaction status.

in the case method "getCode" return a empty value, method "getResponseCode" dont exists.