Adds custom variable for dynamic return URL with WorldPay offsite

This will let merchants using WorldPay offsite redirect customers to their own dynamic thank you page after a successful purchase.  Merchants who want to use 3d secure have to use this older offsite integration.

WorldPay promotes [using a meta refresh to do this now](http://www.worldpay.com/support/kb/bg/paymentresponse/Payment_Response.html#pr5402.html).  I've been talking with their support team.  This will let merchants add something like "meta http-equiv= (http-equiv=) "refresh" content="3;URL='<WPDISPLAY ITEM=MC_return>'" to their resultC.html page, and that will redirect customers back to the merchant's own 'thank you' page, instead of WorldPay's, after a successful purchase. This will let them use analytics to fully track all aspects of the order.

@melari @jduff