LOS ANGELES (AP) - Two former presidents and the widow of Lyndon B. Johnson are calling on the History Channel to investigate a documentary it aired alleging President Johnson was involved in the Kennedy assassination. Lady Bird Johnson said the film "falsely and irresponsibly" accuses her husband of conspiring to kill President Kennedy. No accusation made against Johnson "has hurt as painfully," the 91-year-old former first lady said in a Jan. 29 letter. Her husband died in 1973. Copies of her letter were sent to the chief executives of three companies that own A&E Networks, which includes the History Channel....