Qatar mediates two-day extension of truce between Israeli and Hamas forces in Gaza; Egypt also involved; no immediate comment from Israel; extension may include release of 20 Israeli hostages seized by Hamas in October assault; 60 Palestinian prisoners to be freed in exchange; White House hopes Americans among hostages released; truce marks longest break in fighting since October; over 13,000 killed in Israeli military response; 58 hostages released by Hamas; 117 Palestinians freed by Israel; 184 hostages still held, including 14 foreigners and 80 Israelis with dual nationality.