BAGHDAD (Reuters) - A high-profile U.S.-sponsored trade fair for companies rebuilding Iraq was postponed on Thursday, a day after the grisly killing and mutilation of four American contractors deepened fears that security was worsening. Iraq's U.S. governor, Paul Bremer, vowed to hunt down those responsible for ambushing the contractors in the restive city of Falluja, and those who then torched the corpses and dragged them through the streets before hanging them from a bridge. "Yesterday's events in Falluja are a dramatic example of the ongoing struggle between human dignity and barbarism," Bremer said in a terse address at a police...