The U.S. economy continued to build momentum in August, with employers adding 151,000 workers to their payrolls, the Labor Department reported early Friday. The unemployment rate held steady at 4.9 percent for the third consecutive month, and average hourly wages rose 0.1 percent. August is seen by economists as a difficult month to measure, as summer vacations and the new school year throw off companies’ usual hiring decisions. Federal Reserve officials and the financial markets are closely watching the jobs numbers for clues regarding an expected increase in interest rates.