GAO, Mali (Reuters) - Islamist insurgents launched a surprise raid in the heart of the Malian town of Gao on Sunday, battling French and local troops in a blow to efforts to secure Mali's recaptured north. Local residents hid in their homes or crouched behind walls as the crackle of gunfire from running street battles resounded through the sandy streets and mud-brick houses of the ancient Niger River town, retaken from Islamist rebels last month by a French-led offensive. ...