payment: add getPaymentMethodDetail for plugin

This call is generic enough to have it in the PaymentPluginApi.

Signed-off-by: Pierre-Alexandre Meyer <pierre@ning.com>
payment: fix NPE in TestPaymentMethodProcessorRefreshWithDB

Signed-off-by: Pierre-Alexandre Meyer <pierre@ning.com>
payment: fix regression in PaymentMethodProcessor

00dc2dbadd8ec3e2cdd524b9e683f955dac67a8f introduced a regression as
payment methods were tied to plugins, not instances.

I'm not sure though if using the OSGI service name in the long run is
the way to go. Could we instead auto-discover at runtime which plugin
is supposed to handle a given payment method?

Signed-off-by: Pierre-Alexandre Meyer <pierre@ning.com>