Update PaymentServer
- Merge pull request #79 (closed) from PrivateStorageio/78.tests-in-nix-packaging
- Merge pull request #80 (closed) from PrivateStorageio/77.payment-metadata-handling
- Merge pull request #83 (closed) from PrivateStorageio/82.currency-parsing
- Merge pull request #85 (closed) from PrivateStorageio/84.currency-and-price