BUGFIX: 3-arg subtraction was not implemented

The following change allows use of sub like
```py

sub(5, 4, 1) #0
```

We promised customers that we would have this in our initial release,
so this is a bug, not an enhancement

src/calc_test.py

@@ -17,6 +17,9 @@ def test_sub_2arg(self):
         # Make sure 4 - 3 = 1
         self.assertEqual(sub(4, 3), 1, 'subtracting three from four')
 
+    def test_sub_3arg(self):
+	self.assertEqual(sub(4, 3, 1), 0, 'subtracting three and one from four')
+
 
 if __name__ == '__main__':
     unittest.main()