Set a unique key for each order
Set redis state to show the invoice has been sent and don't requeue sent orders
Fix existing tests
Add new test to check old invoices are not requeued
Explain why we're mocking away the invoice_sent function when deleting invoices
Rename `redis_key` to `invoice_uid`
Add `invoice_uid` to doc block
Merge pull request #281 from theodi/bugfix-73-xero-rate-limit

Make sure orders that have been invoiced are not requeued