(LENOX, Mass.) – The Boston Symphony Orchestra begins its final weekend of the 2011 Tanglewood season Friday, August 26, with its first-ever performance of George Gershwin’s great American masterpiece, the blues-and-jazz-inflected Porgy and Bess, which examines African-American life in the South during the 1920s. The great Itzhak Perlman joins the orchestra on Saturday, August 27,…