By Brad Brooks and Rodrigo Viga Gaier RIO DE JANEIRO, Aug 7 (Reuters) - Portugal's education minister was robbed at knifepoint on Saturday at the Olympic lake where rowing events are taking place, but escaped unharmed, authorities said. Education Minister Tiago Brandao Rodrigues and an aide were walking near the event area when they were assaulted. Rio has dramatically lowered its crime levels in recent years, but there has been a spike in murders and assaults in recent months. More...