invoice-generate: Add tests for get_all_active_account_ids, first_day_of_next_month and last_day_of_this_month.
invoice-generate: Support for tax rates other than 0.21. Still no support for multiple tax rates on the same invoice, but this is a start.
invoice-generate: In tests, change the default account ID so it doesn't conflict with the account we create
invoice-generate: test for generate_invoice() if there's nothing to invoice
invoice-generate: Add add_queue_itemline() method and tests for queued itemlines.