Add Spree::Payment#editable?

* This method returns true if the payment amount can be edited
Change Spree::Api::PaymentsController#update to update editable payments
Change payments to be editable when the state is checkout or pending
Merge pull request #26 from MountainRoseHerbs/2-3-stable-mrh-editable-payment

Change payments with "checkout" status to be editable