PITTSBURGH (Reuters) - Bolstered by other world powers, U.S. President Barack Obama demanded on Friday that Iran come clean about its nuclear program or face "sanctions that have bite" after the disclosure of a secret uranium enrichment plant. Issuing a stern warning to Iran at the end of a Group of 20 summit in Pittsburgh, Obama pointedly declined to rule out the military option in dealing with the bitter standoff with Tehran but insisted he preferred to resolve it diplomatically. Obama stood earlier in the day with the leaders of Britain and France in accusing Iran of working in secret...