Transact doesn't work for token-requests anymore

Hi :-)

I'm currently trying to integrate PayMill into a project I'm working on and noticed an issue with the transact method. Since the refactoring it seems like it requires either the payment-parameter or its replacements but doesn't work anymore with at least the token parameter.

I've attached some changes to that checking logic and (since I couldn't find any unittests so far) added some for this based on py.test and tox if this is alright with you.