Amend BIP72 by an "h" parameter, which contains a hash of the…

…PaymentRequest message that is fetched via the "r" parameter.

The hash is meant to link the trust anchor (e.g. the QR code) to the payment request message in a secure way. This will solve the problem several apps are comparing address+amount fields as a workaround instead, preventing some advanced BIP70 usecases. When these apps read a matching hash, they need not compare any of the other fields.

Thanks to Julian Haight for helping with the standard.