Adds Void Payment abstract to PaymentGatewayMethodBase and implements in CashPaymentGatewayMethod.

First pass at the fixing the omission pointed out by Matt Muller.  I included a new field on the merchPayment table "voided" ... which will need to be added to your databases.  Will work on an automatic update strategy soon ...

This also updates the Umbraco 7.1 dependencies to 257 nightly.