Cart모델 내 쿠폰 할인 메서드 추가

[purelledhand]

1.getAvailableCouponItems

2. discountPrice(coupon)

Switch Coupon.type
Rate: return number(getacitems.reduce(item*rate))
Amount: return
Getacitems.reduce sum, item < amount ? 전자 : amount

3. PaymentPrice()
   totalprice - discountprice