WASHINGTON (Reuters) - Retail sales contracted in March for the second time in three months, a sign the American economy may have stumbled at the end of the first quarter. Retail sales fell 0.4 percent during the month, the Commerce Department said on Friday. That was below analysts' expectations that sales would be flat. Readings for retail sales have been volatile so far this year, making it difficult to know whether the weakness in March was due to a tax hike that went into effect at the start of the year or to temporary factors related to the weather. ...