JERUSALEM, Nov 21 (Reuters) - What began as an everyday trip to school for many children on Jerusalem's number 20 bus ended in carnage on Thursday after a Palestinian suicide bomber posing as a commuter boarded and blew it up. One of the 10 dead, a 14-year-old girl, died on arrival at Jerusalem's Hadassah hospital. "She was fatally wounded. There was really nothing we could do for her," Dr Shmuel Shapira, deputy head of the hospital, told Israel Radio. One witness said he had heard survivors screaming "Mamma, Mamma" from the twisted wreckage of the bus, whose windows were...