Create task to clean up failed/abandoned payments.

[bummzack]

This issue replaces: #66

With the 2.0 release, Payments will have special "Pending" states, such as PendingAuthorization, PendingPurchase, PendingCapture, PendingRefund and PendingVoid. If a payment fails, it will remain in that state…

There should be a task that will clean up these failed payments, probably after a certain amount of time has passed (eg. if a payment is still in a "pending" state 3 days later, it's safe for removal). The removal delay should be configurable…