By Richard Knox Happy Birthday, America! Two hundred and fifty years is an impressive landmark, although the U.S. of A. is still a youngster among nations. Compare our 1776 Declaration with the 1215 Magna Carta, which limited the powers of the English king (ahem!). Or the Roman Empire, which lasted 503 years. Not to mention India, Egypt or China, whose history is dated in the thousands of years. Chorale is known for concerts that speak to the Zeitgeist, I wondered how Dan Per