Combine first 5 ingredients, stirring with a whisk. Pour mixture into freezer can of an ice-cream freezer, and freeze according to manufacturer's instructions. Stir 1/3 cup graham crackers into ice cream. Spoon ice cream into a freezer-safe container, and cover and freeze for 1 hour or until firm. Sprinkle each serving with 1 teaspoon graham crackers. Garnish with lime wedges, if desired.