Federal prosecutors today kicked off the long-awaited trial of accused Boston Marathon bomber Dzhokhar Tsarnaev with a devastating chronology of his alleged actions on the day of the bombings that killed three and wounded 260 — as defense attorneys freely admitted “it was him.” “He decided to place his bomb right behind a row of children... one of them was Martin Richard,” federal prosecutor William Weinreb said during opening statements at the John Joseph Moakley United States Courthouse in Boston. “When he was a safe distance away, he detonated his bomb ... Some bled to death on the sidewalk as...