Finalize order/invoice/payment/account models

[benjaoming]

This issue does not describe the solution. The issue is to describe and consolidate the correct state-of-the-art handling of orders, invoices, refunds, and account balance.

@michkoz has indicated that it's hard to approach solutions and designs to user stories when models to support views/templates are not finalized.

I agree, this is part of the developer-friendly milestone.

[benjaoming]

I've marked it "quick or easy" with the understanding that if we get behind the discussion and flesh out the design of the models, the implementation itself should be trivial :)