Combine all ingredients in a small saucepan; bring to a boil. Reduce heat, and simmer 10 minutes. Remove from heat; let stand 10 minutes. Pour mixture into a blender; process until smooth. Strain through a sieve over a bowl, and discard solids. Pour into an 11 x 7–inch baking dish; cool. Cover and freeze for 45 minutes; scrape with a fork. Freeze. Scrape every 45 minutes until completely frozen (about 6 hours). Remove from freezer; scrape with a fork until fluffy.