GENEVA (Reuters) - George Clooney has raised hundreds of thousands of dollars for U.S. presidential candidate Barack Obama by headlining an exclusive event for Americans in Geneva, one of the world's most affluent cities. The Hollywood actor and director, fresh from the world premiere of the Coen brothers' comedy "Burn After Reading" in Venice, slipped without fanfare into the Swiss city on Tuesday for his first appearance on behalf of the Democratic nominee. Some 170 contributors have paid $1,000 a head to attend a cocktail party with Clooney at a museum in Geneva's Old Town on Tuesday evening. And 75...