Changing status to High from Critical, since we now have a way to cross-reference PGP
orders to Auth.net transactions via the 'Invoice#' field on the Auth.net side.

I still think we should persist this on the PGP side as well, so we have a way to
know who purchased what (for anonymous orders that have no customer id on the
Plone/PGP side.)

Ok the last four digits magic is done, in getpaid.authorizenet by kapilt, in
getpaid.paymentech by lucielejard and in getpaid.nullpaymet by javimansilla.
The recipe is simple:
#We define the annotation key
LAST_FOUR = "getpaid.PAYMENT_PROCESSOR_NAME.cc_last_four"
#Then in our code we store the last four by getting the annotation and setting the
#corresponding key
annotation = IAnnotations( order )
annotation[ LAST_FOUR ] = CREDIT_CARD_LAST_4_DIGITS

javimansilla made me realize that this is not optimal (the case proposed was that you
change the payment processor and yet you have the key with the other one's name),
perhaps we should find a way to have a generic getpaid.cc_last_four and a way inside
it to identify the payment processor, i would suggest a dict inside the cc_last_four
key but I am not familiar enough with the internals of this.

Started at PloneConf sprint. 

We've persisted these fields to the getpaid order and modified the Orders list page
(Orders tab in GetPaid Site Setup admin screens), so that an orders administrator can
reconcile getpaid orders to the processor's orders (cross-referencing of order/trans
IDs to card names, last4, etc.)  Admins can do a lot more reconciling without logging
into processor's web admin UI.