DONETSK Ukraine (Reuters) - East Ukraine's rebel stronghold Donetsk was pummeled on Sunday by the heaviest shelling in a month, and the OSCE said it spotted an armored column of troops without insignia in rebel territory that Kiev said proved Moscow had sent reinforcements. A two-month-old ceasefire to end a war that has killed 4,000 people has appeared shakier than ever in the past few days, with both sides accusing the other of having violated the terms of the peace plan. ...