Add support for ePayment API

[zaporylie]

New, consolidated ePayment API https://github.com/vippsas/vipps-epayment-api is to eventually replace Vipps eComm API which is currently supported by this library.

ePayment API is currently not finished (one cannot initiate payment that way) but at the same time is required for post-purchase handling of orders paid with Vipps Checkout (#44)

The expectation is that ePayment API will be implemented independently from eComm API (currently accessible via https://github.com/zaporylie/php-vipps/blob/2.x/src/Vipps.php#L53) and exposed as Vipps::ePayment()