gonzalo higuain gave visitors the lead after 15 minutes with a smart finish .
there was a suspicion of handball as the argentine brought the ball down .
marek hamsik doubled lead on 23 minutes , higuain the provider .
slovakian midfielder added his second and napoli 's third in second half .
manolo gabbiadini added a fourth for visitors moments after coming on .
former arsenal striker nicklas bendtner pulled one back for the hosts .