WASHINGTON (Reuters) - Democratic presidential candidate Barack Obama's grandmother died of cancer, he said in a statement on Monday, a little more than a week after Obama interrupted the White House race to say goodbye to her in Hawaii. "It is with great sadness that we announce that our grandmother, Madelyn Dunham, has died peacefully after a battle with cancer," Obama said in a joint statement with his sister, Maya Soetoro-Ng. "She was the cornerstone of our family, and a woman of extraordinary accomplishment, strength, and humility." Dunham helped raise Obama from the age of 10 while his mother was...