Unique reference id for invoice

In prepareTemplate currently, the invoice id in the returned object is set to the createdAt-time stamp. This is probably OK but could be problematic if invoices happen to be created simultaneously.

The number in question is a reference id for the invoice and is as such required to be digits only.

Two spontaneous solutions:

1. Find a hash-generator without collisions that only generate numbers.
2. Find some way to make sure that two invoices never get the same id, e.g. by making the creation of n invoices take n ms.