Log form output to improve debugging

[joshnuss]

When debugging issues it would be helpful if we had a log of what form/inputs were generated.

Possible approaches:

1. Add an option to #payment_service_for, ie: payment_service_for(..., log: true) and call Rails.logger
2. Update #payment_service_for to bubble up an event that can be handled by the host app. ie: ActiveSupport::Notifications.instrument("#{service}.form", html)