Refactor Adaptive Payments.

New class `Payment_Adaptive_Simple` is introduced as a base class for
simple, parallel and chained payments.

`Payment_Adaptive` is an abstract base class for all Adaptive Payments API calls.

`Payment_Adaptive_Refund` is introduced for the Refund API.
It extends `Payment_Adaptive` similar to `Payment_Adaptive_PaymentDetails`.

Converting receivers to receiverList is extracted into `Util::receiver_list` with
support for accountId.

Added a bit more test coverage, but new code and @covers annotations are reducing
the code coverage percent.