By Martyn Herman RIO DE JANEIRO, Aug 19 (Reuters) - American Alise Post's joy at winning a silver medal in the women's BMX final was tempered shortly afterwards on Friday as Australian husband-to-be Sam Willoughby finished off the podium. Minutes after Post crossed the line second behind winner Mariana Pajon, she watched Willoughby launch off the start ramp bidding to go one better than the silver he won in London. "But it's BMX and its one of those sports where anything can happen. More...