Merge pull request #1479 from stloyd/bugfix/pawel_mail

Fix Paweł mail =)
Support per-user limits on coupons. This restricts users from placing multiple
orders with the same discount.  This is a commonly used feature in magento for sites
that have a max $ limit on their coupons.  Users bypass this by placing multiple orders.
Magento however doesn't force you to be logged in during the time of ordering, so it's easly bypassed
that system.