I hardly ever take advantage of the Kennedy Center’s Millennium Stage, which offers free performances every day in a wide variety of art forms. On a whim, I went yesterday to check out Jayme Stone , a Canadian banjo player who explores worldwide folk traditions. It was good! Stone wasn’t a banjo virtuoso (not that I’m complaining), but he had a killer band backing him up on interesting material. I was entertained and even amused at times, as the line-up generally took the form of a typical blueg…