KANO, Nigeria (Reuters) - Nigerian forces killed two Islamist gunmen in a shootout and arrested five others after their checkpoint in the north's main city of Kano was attacked on Thursday, they said. Kano has seen a spate of shootings this week attributed to the Islamist militant group Boko Haram, based loosely on the Afghan Taliban. The group killed hundreds last year in a campaign to impose sharia, or Islamic law, on religiously mixed Nigeria. A crackdown last year weakened it but also pushed it into new areas, and it remains the biggest security threat to Africa's top oil producer. ...