By Pedro da Costa and Alister Bull WASHINGTON (Reuters) - The Federal Reserve's monetary stimulus is helping the economy recover but the central bank needs to see further signs of traction before taking its foot off the gas, Fed Chairman Ben Bernanke said on Wednesday. In testimony that offered little sign he is ready to retreat from the Fed's latest round of bond buying, Bernanke emphasized the high costs of unemployment and inflation that continues to run below the Fed's target. ...