Validate credit card type and use specific cvv for each credit card type
Authorize.Net: Added duplicate_window. x_duplicate_window tells us, in seconds, how much time we should check for duplicates after a transaction is submitted
PayPal: Added PayPal Website Payments Pro gateway
XMLGateway: Allow passing of extra headers and abstracted envelope property (to allow for easier SOAP envelope integration)
PEP 8 cleanups