Removing Coupon from Subscription

Previously, the coupon was added to both the Customer AND Subscription objects. This caused the coupon to be redeemed twice, which presented problems for coupons with a limited redemption count. Now, the coupon is only added to the Customer object.

Signed-off-by: Adam Engebretson <adam@enge.me>
Merge pull request #124 from adamgoose/master

Removing Coupon from Subscription