Add a test for hexidecimal parsing

lib/koans/16_integers.ex

@@ -34,4 +34,8 @@ defmodule Integers do
 
     assert string_digit == ___
   end
+
+  koan "The meaning of life in hexidecimal is 2A!" do
+    assert Integer.parse("2A", 16) == {___, ""}
+  end
 end