Offline payments / PaymentACK via BT

[ghost]

Possible on iOS? @javgh

[javgh]

Offline payments not, as we need access to the backend server, but PaymentACK via Bluetooth should be possible using Cordova. Although if you have an internet connection anyway, that extra ACK via Bluetooth might not be all that useful. In any case, PaymentACK is part of payment protocol, so it would probably make sense to first implement that.