anchor.diff:        expect(amortized.payments[5].interest, 0.24);
         expect(amortized.payments[5].principal, 28.39);
       });
  +    
  +    test('Payoff', () {
  +      var debt = new Debt('Test1', 100.0, .1, 25.0);
  +      var amortized = new Amortization(debt);
  +      expect(amortized.payoff, 100.83);
  +      amortized.addPayment(25);
  +      expect(amortized.payoff, 76.46);
  +    });
     });
   }
positive.code:       var debt = new Debt('Test1', 100.0, .1, 25.0);
        var amortized = new Amortization(debt);
        expect(amortized.payments.length, 0);
        amortized.addPayment();
        expect(amortized.payments.length, 1);
        expect(amortized.payments[0].interest, 0.83);
        expect(amortized.payments[0].principal, 24.17);
  
negative.code:       var debt = new Debt('Test1', 100.0, .1, 25.0);
        var amortized = new Amortization(debt);
        expect(amortized.payments.length, 0);
        amortized.amortize(25);
        expect(amortized.payments.length, 5);
        expect(amortized.payments[4].interest, 0.02);
        expect(amortized.payments[4].principal, 2.11);
  