By Dave Graham IGUALA Mexico (Reuters) - Authorities have found mass graves with the charred remains of up to 20 people in the restive southern Mexican state of Guerrero, at a time when police are scouring the area for nearly four dozen missing students after a rash of violence. The remains were buried on a hillside up a rocky dirt track on the outskirts of Iguala in six suspected graves, which were still fresh, a local official said, speaking on condition of anonymity. Investigators discovered the burned remains, which were put into bags, two officials said, asking to remain anonymous. ...