Add one-to-many Payment relationship to Order.
Deprecate the old getPayment and setPayment methods.
Fix comments.
Update payment listener and state resolver to loop through all payments.
Fix comments.
Fix comments.
Fix specs.
Add new tests to cover payment states.
Add getLastNewPayment.
Fix comment.
Merge pull request #1203 from kayue/feature/payment-order

Multiple payments support.