Add more methods to CompletePurchaseResponse

Allow to get some more information about the response, like the transactionId/orderId/AuthorisationId for manual checking.
There are some more methods available sometimes, you want me to add them as well? https://www.rabobank.nl/images/integratiehandleiding_rabo_omnikassa_en_version_7_1_april_2014_final_2_0_29637101.pdf#page=17

Todo:
 - [x] add tests