ATHENS (Reuters) - Greeks went to the polls on Sunday in an election that could decide whether their heavily indebted country remains in the euro zone or heads for the exit, potentially unleashing shocks that could break up the single currency. In an election fought over the punishing austerity package demanded by international lenders as the price of keeping Greece from bankruptcy, opinion polls showed the radical leftist SYRIZA party, which wants to scrap the deal, running neck and neck with the conservative New Democracy, which broadly backs it. ...