US Senator Elizabeth Warren joined Boston Mayor Thomas M. Menino today to press for stricter gun control and throw her support behind three Senate bills that are expected to be filed next week in a flurry of legislative responses to 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 mansion where Menino is convalescing after his recent illness.