ABC News has learned that Richard Holbrooke, the US Special Representative to Afghanistan and Pakistan, has died. On Friday, Holbrooke was rushed to the hospital with a torn aorta. He went through more than 20 hours of surgery. Earlier this evening, speaking at the US State Department, President Obama sang Holbrooke's praises and called him "a tough son of a gun." Holbrooke, 69, was a former ambassador to the United Nations and served as chief negotiator at the Dayton Peace Accords, which ended the war in Bosnia. The New Yorker's George Packer wrote a nice story about Holbrooke last year,...