(Adds quotes) By Martyn Herman RIO DE JANEIRO, Aug 15 (Reuters) - Mark Cavendish finally got his hands on an Olympic medal when he took silver in the omnium on Monday, and while the British rider said he was happy to be on the podium he conceded his impressive collection of honours is incomplete without a Games gold. The former world road champion, who has also won the Tour de France's (2011) points classification title, the Milan-San Remo (2009), and is a three-times world Madison champion on the track, had put everything on the line for Rio. More...