Add support for manual offline payment

[gnoff]

Recurly offers a beta feature for recording an offline payment through their api

this should trigger a successful_payment webhook as opposed to the markSuccessful function which simply results in an invoice_closed webhook

api documentation here: https://dev.recurly.com/docs/enter-an-offline-payment-for-a-manual-invoice-beta

I can maybe do a PR if maintainer has interest

[zero-master]

Is there any update on this? I am interested.