Set coupon code to be unique. Add expire time.
Add CouponManager with get_valid_coupon method.
Validate coupon upon invoice creation.
Add coupon API for coupon validation.
Fix id naming. Use dash instead of underscore.
Add hidden global class.
Validate coupon on cart page.
Merge pull request #11 from gregwym/coupon

Add coupon validation on cart page and during checkout process. Solve #7.