[RFR] Sepa Credit Transfer

Hello Dmitri,

We also needed to do sepa credit transfer. As I was already using your library, I forked it to add this functionality. 

My changes introduce a backward compatibility break. Now PaymentInfo can be used for both Payment Methods "Direct Debit" or "Credit Transfert".  The Payment Method is automatically set when you use addDirectDebitTransaction or addCreditTransfertTransaction. But is not when you don't add any transaction (that was the case in unit tests). I think that there may have other breaks but you would have to do minor changes in your code to make it work.

Please have a look and tell me what you think.