TRIPOLI (Reuters) - Three days of clashes between rival militias in southern Libya spread to the centre of the country's fourth largest city Sabha on Tuesday despite the deployment of army troops trying to stop the violence which has so far killed nearly 50 people. The clashes highlight the problems the government faces in imposing its authority following the overthrow of Muammar Gaddafi last year. Fighting between gunmen from Sabha and those from the Tibu ethnic group had reached the centre of the city, said Ibrahim Misbah, a doctor at the main hospital. ...