Invoicing

Handle error properly while calling Automaton, perfect to pass them untouched with the help of latest woody release.
Better and easier to compehend flow control in machines.
More familiar flow control handling of machines, e.g. catching and wrapping thrown exceptions.
Explicit stage denotion in the invoice machine?
Submachine abstraction and payment submachine implementation.
Invoice access control.
Proper behaviours around machines w/ internal datastructures marshalling, event sources and dispatching.

Tests

Add generic albeit more complex test suite which covers as many state transitions with expected effects as possible.