Smart online shoppers always search the Web for a coupon before they make a purchase. RetailMeNot, one of the leading coupon Web sites, is also trying to offer coupons for mall shoppers via their cellphones.The idea is the same as the paper coupons that people used to clip out of newspapers. But now they are delivered by RetailMeNot’s new iPhone app, which is awaiting approval from Apple, when a shopper walks near one of hundreds of malls across the country.RetailMeNot, which is owned by WhaleShark Media, the operator of several online coupon sites, realized that 20 percent of its traffic was coming from mobile devices, so it assumed people