WASHINGTON (Reuters) - A computer hacker got into the U.S. agency that guards the country's nuclear weapons stockpile and stole the personal records of at least 1,500 employees and contractors, a senior U.S. lawmaker said on Friday. The target of the hacker, the National Nuclear Safety Administration, is the latest agency to reveal that sensitive private information about government workers was stolen. The incident happened last September but top Energy Department officials were not told about it until this week, prompting the chairman of the House of Representatives Energy and Commerce Committee to demand the resignation of the head of...