The Rev. Richard Shahan,long suspected in the killing of his wife, was arrested on a murder warrant as he attempted to board a flight for Germany.    Authorities say Shahan was taken into custody at the Nashville airport on New Year's Day.   Shahan had informed people that he was going to Germany as part of a mission project.  But Homewood police say Shahan intended to flee to Russia, after arriving in Germany.  Shahan was on leave from the First Baptist Church of Birmingham after his wife was found murdered at the couple's home last July.    Shahan had long been suspected in the killing.     Police did not say if new evidence led to his arrest.