Generate signatures based on the HPP method for the checkout form

I'm failing to get a signature that will be accepted against the test credentials we have. I can't tell if that's because the credentials are not actually valid or the method of generating the signature isn't quite what they expect. The documentation is sparse and shoddy, so I believe we'll need to communicate with someone at SnapPay to get to the bottom of it.

(Additionally, their examples are all based on JavaScript even though one of the parameters is supposed to be a secret key ... we can't very well be putting that into the DOM for use by a script in that case.)