This summer, as the World Cup heads to South Africa, close to a billion people will be watching "the beautiful Game." ANNIE LEIBOVITZ photographs some of the reigning gods of the stadium, while A. A. GILL explains why football—please don't call it soccer—is the most important sport in history: a lingua franca for 204 countries, an expression of national identity, and a powerful link between multi-millionaire athletes and the man on the street