payment method

from https://github.com/elf-pavlik/currency/issues/3#issuecomment-217305124,

We also have the whole 'payment method' mess, which only relates to currencies and no other types of digital resources - cash, check, credit card, direct, transfer, payment gateway (paypal, satoshipay), or even https://www.w3.org/Payments/

here's a dedicate issue regarding "payment method".