Renamed `NotifincationURL()` to `NotifyUrl()`
Added a new `Transaction` class and `TransactionInterface` contract. I'm not sure if this is right way to go yet.
Removed all the transaction specific stuff from `PayXML` class.
Added `Card->type()` which allows setting the card type, but think I'm going to remove most of the setters. Objects must be populated through their constructors.