By Jeremy Walton   On February 14th, 1989, the Ayatollah Ruhollah Khomeini sent what surely must have been one of the blackest Valentine’s greetings of all time to novelist Salman Rushdie.  Invoking somewhat dubious legal and theological authority—as a Twelver Shi’a, Khomeini could hardly claim to speak for all of the world’s Muslims—he called for Rushdie’s death on...