* Refactor logic for calculating due amount of invoices
* Remove Money type from the accounting
* Fix tests
* Fix a corner case
* fix bug
* Rename PaymentCurrency to Currency
* Fix bug
* Rename PaymentCurrency -> Currency
* Payment objects should have access to the InvoiceEntity
* Set Currency USD in tests
* Simplify some code
* Remove useless code
* Simplify code, kukks comment
