Added support for percentage values.

In addition to a money value, the voucher subsystem now allows for a percentage of the total order.

Added support for usage limit per voucher.

A total number of times a voucher, accross all customers. This is great for marketing campaigns where the campaign driver is 'First 50 customers to order get this discount'

Added support for usage limit per customer per voucher.

We have added this voucher feature to allow the parlor owner to limit usage to one voucher per customer.