A policeman's expletive-laden tirade at a teenage driver is under investigation by the independent police watchdog after being caught on tape.The officer called the 16-year-old a "dickhead" and "idiot" and repeatedly used the f-word during the stop, apparently because he believed the boy had pulled the finger at him. He was so close during the outburst some of his spittle landed on the young man's face. "I thought he was going to hit me," said Dylan Vaughan, the driver. "Every time I see a pol...