Allow a store owner to configure when tax (VAT) should be applied. Before or after reward points (discounts, etc)

[AndreiMaz]

VAT is currently calculated on the sub total before reward points value has been applied. UK policy requires VAT to be applied to the actual amount paid.

We have invoices where, because of reward points, the customer has paid £0 for their order, and yet the invoice shows a value for VAT.

Source: http://www.nopcommerce.com/boards/t/44583/error-in-vat-calculation-when-reward-points-are-used.aspx

also see pull request: #1992