Provides payment processors and settings screens for other plugins to use to process payments.
PaymentProcessor::addProcessor($processor) -- ties an instance to a slug
aPaymentProcessor->process($request, $responseURL) -- TODO: make it take a function instead of a URL. Do the actual processing, get the result.
PaymentProcessor::getProcessor($slug) -- Get an instance by slug
PaymentProcessor::getProcessors() -- get the list of registered processors for user selection