If you are lucky enough to have the President’s Day holiday off work and you manage to wake up before 5 p.m., it may be a good chance to go down to the National Mall and look at some art. The National Gallery of Art’s new exhibit, Jasper Johns: An Allegory of Painting, 1955-1965, takes a focused look at four themes in the artist’s work during an important decade in American art. Johns is one…