WASHINGTON (Reuters) - Besieged by unflattering stories about the launch of President Barack Obama's healthcare program, the White House saw a news report that it wanted to swiftly knock down. It was from NBC, which said that Obama had overpromised when he said Americans who liked their insurance could keep it, and that the president knew that many people would see their coverage change. White House officials quickly began firing off a barrage of tweets on Twitter, which has become one of the administration's most potent and relied-upon weapons in trying to shape public opinion and media reports. Josh Earnest,...