In a large bowl, combine couscous, oil, salt and pepper. Pour water over mixture and cover. Let stand 5 minutes or until water has been absorbed. Uncover and fluff with a fork. Let cool completely. In a food processor, finely chop parsley, mint and chopped scallions. Add to couscous along with tomatoes and lemon juice. Toss to blend. Gently stir in feta. Line pita halves with lettuce. Stuff with couscous mixture.