Account Invoice Zero

Invoices with a amount of 0 are automatically set as paid.

When an invoice has an amount of 0, OpenERP still generates a receivable/payable move line with a 0 balance. The invoice stays as open even if there is nothing to pay. The user has 2 ways to set the invoice as paid: create a payment of 0 and reconcile the line with the payment or reconcile the receivable/payable move line with itself. This module takes the latter approach and will directly set the invoice as paid once it is opened.