Fixed error thanks to Travis and unit testing :).
Store transaction ID and action URL only in payment, this is no longer part of the gateway class/object.
Use payment ID instead of the order ID, not all extensions have an 'order' or 'order id'.

Store transaction ID and action URL only in payment, this is no longer part of the gateway class/object.
Added some get functions for amount and currency.
Auto load amount and currency of the payment.
Fixed get amount from input.
Removed some test functions.
Improved support for the Membership plugin.