NEW YORK (Reuters) - Look for U.S. stocks to drift higher this week as optimism about the economy and earnings continues. But key data on the employment picture, auto and retail sales could trip up the market's momentum. The week is likely to get off to a slow start as investors return from a long holiday weekend. U.S. stock, bond and oil markets will be closed for the Memorial Day holiday on Monday. But activity will likely pick up when a slew of economic data hits traders' screens. Strategists thought the market's mood was improving going into June. "I would...