Add a new parameter: DEFAULT_PAYMENT_METHOD

* Make it possible to have a default payment method to select on sale checkout
* When calling .set_client, be less intrusive. If there's no client or it
  doesn't have enough credit, select the default method instead of selecting money
* Remove arg 'methods'. It wasn't being used by the class neither was passed by
  any of the callsites

Change-Id: Ifb1455e024436e731ac7a31f0f9293fda4b96d00