Accept payment on backend

Useful patch, when the payment backend ``PayOnDeliveryBackend`` is used. It can also be used in other circumstances.

Set order to status to be COMPLETED, if a payment was received through wire transfer or in cash by surface mail. If the vendor accepts this payment and adds it to the backend, then the orders status is set to COMPLETED and the appropriate signal is sent.

I also fixes some spelling and PEP8 issues in a separate patch.