SYDNEY (Reuters) - Asian share markets started on a cautious note and gold gained on Monday as images of riots in burning U.S. cities unnerved investors already tense over Washington's power struggle with Beijing.E-Mini futures for the S&P 500 retreated 0.5% in early action, while gold rose 0.77% to $1,739 an ounce . Oil prices also slipped, while sovereign bonds picked up the usual safe-haven bid.MSCI's broadest index of Asia-Pacific shares outside Japan edged up 0.2%, as did Japan's Nikkei..