MEXICO CITY (Reuters) - Enrique Pena Nieto took over as Mexican president on Saturday, offering a shot at redemption for the party that shaped modern Mexico if he can bring about an end to years of violence and economic underperformance. Returning the centrist Institutional Revolutionary Party, or PRI, to power after a 12-year hiatus, the 46-year-old Pena Nieto aims to use a recent improvement in the economy's fortunes to spark faster growth. ...