Implement payment tracking for subscriptions

This tracks successful and failed invoice payments as `Payola::Sale` instances.
Every time you create a subscription Stripe creates an invoice immediately and attempts to pay it. If your plan has a trial period (or if you override the trial in `Stripe::Subscription#create`), then Stripe will create a zero dollar invoice and "pay" it successfully. Technically you can create a customer without a card and subscribe them to a trial plan and Stripe will still create these zero dollar invoices. In Payola-land, we don't really care about these zero dollar invoices, at least in this context, so we just ignore them entirely.