Senator Elizabeth Warren joined Mayor Thomas M. Menino on Friday to press for stricter gun control and throw her support behind three Senate bills that are expected to be filed next week in the wake of the massacre in Newtown, Conn. “We owe it to our children to start,” Warren said at a news conference at the Parkman House, the city-owned Beacon Hill residence where Menino is convalescing after a lengthy illness. On Dec. 14, a gunman with an assault rifle and two pistols killed 20 first-graders and six adults at the Sandy Hook Elementary School. Warren said she will...