GEVGELIJA, Macedonia (Reuters) - Thousands of migrants stormed across Macedonia's border on Saturday, overwhelming security forces who threw stun grenades and lashed out with batons before apparently abandoning a bid to stem their flow through the Balkans to western Europe. Some had spent days in the open with little or no food or water after Macedonia on Thursday declared a state of emergency and sealed its borders to migrants, many of them refugees from war in Syria and other conflicts in the Middle East. But by nightfall on Saturday, thousands had crossed the frontier, milling around the border town of...