add various helper accessors on SagePay ServerAuthorizeResponse and ServerCompleteAuthorizeResponse including phpdoc info about the information in the fields
OmniPay\SagePay\Message\Response::getTransactionReference() didn't return
anything when TxAuthNo wasn't available. However, when using
ServerAuthorizeRequest to send a ServerAuthorizeResponse, we need to grab the
transactionReference so that we have the SecurityKey component in
ServerCompleteAuthorizeRequest when we handle the SagePay notification.
So I've changed  OmniPay\SagePay\Message\Response::getTransactionReference()
to return a transactionReference whatever the circumstances. If a component
it usually stores isn't available, it will store it as null.