[orders] unified interface for all possible payments.
wip2
[order] simplify things. remove money, currency, client classes.
[order] get rid of BaseCaptureOrderAction, intr FillOrderDetails request.
[order] add some tests.
[order] add tests for CaptureOrderAction.
[action] add basic get http request action.
[order] update get-it-started docs of all supported payments.
[order][doc] $registry -> $payum