The Nitty Gritty Dirt Band is celebrating five decades together.  Jeff Hanna, Jimmie Fadden, Bob Carpenter and John McEuen will perform a sold out show at the Ryman Auditorium tonight.  It’ll be taped for broadcast on PBS in 2016, the actual year of the band’s 50th anniversary.  They’re also planning to mark the milestone with a tour and a new album.