payment: allow override of payment method id for createDirectPayment

See https://github.com/killbill/killbill/issues/177.

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>
payment: expose plugin properties in DirectTransaction

See https://github.com/killbill/killbill/issues/185.

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>
DirectTransaction: avoid null properties field

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>