Wirecard gateway: Add card store and purchase/authorize by reference

Wirecard supports the notion of "Recurring Transactions" by allowing
the merchant to provide a reference to an earlier transaction
(the GuWID) rather than a credit card.  A reusable reference (GuWID)
can be obtained by sending a purchase or authorization transaction
with the element "RECURRING_TRANSACTION/Type" set to "Initial".
Subsequent transactions can then use the GuWID in place of a credit
card by setting "RECURRING_TRANSACTION/Type" to "Repeated".

This implementation of card store utilizes a Wirecard "Authorization
Check" (a Preauthorization that is automatically reversed).
It defaults to a check amount of "100" (i.e. $1.00) but this can be
overriden.  In order to reuse the stored reference, the
+authorization+ from the response should be saved by application
code.

Also added is the ability to send a reference GuWID, rather than
a credit card, with a purchase or authorize transaction.

Note: any transaction can generate a reusable GuWID reference by
setting the option :recurring to "Initial".  This functionality
already existed but is now documented.