US STOCKS-S&P 500, Nasdaq slip with commods, Alcoa jumps

* MasterCard rises after earnings reports * Alcoa gains on buyout rumor * Dow up 0.1 pct, S&P down 0.2 pct, Nasdaq down 0.6 pct * For up-to-the-minute market news see STXNEWS/US (Updates to midday...