Purchase collects a payment from user using the specified price,
description, and payment method. This is implemented at the lower layers
using the payments.Invoices interface. There are also additions to
stripecoinpayments stripemock to simulate errors returned from stripe
invoices New and Pay. If an invoice exists with the same description, if
it is a draft, it is deleted and a new one is created and paid. If it is
open, pay it and don't create a new one.
Change-Id: Ic3147434bc44a0777ecbedb3a4ed4c768eb02ea3
