Invoice status

Every Invoice Entity has a marking property that describes its processing stage.

Invoice Status	Description
valid	Valid invoice
sending_started	Outgoing Invoice send process is running
sending_queued	Outgoing Invoice is waiting in queue to be send
sending_error	Sending the invoice was not possible.
sent	Outgoing Invoice has been sent to recipient
sent_error	Delivery to recipient was not possible.
received	Incoming Invoice has been received