Add support for Invoices

This branch adds support for invoices.

Status:

- `anchor-api-migrate` includes a table for invoices
- `anchor-apid` provides `/v1.0.0/invoice` endpoints which serve this data
- un/parsing from JSON works on test data.

Todo:

- synch the invoices into the database (to be added to `anchor-retcon`, not here).

So this is ready for review and merge.
I think a better idom for this (for production code, this is fine), is to not test for the existence of the file and just handle IO exceptions.