add a self-test using doctest.  Results:

- The docstring needs to be a raw string because it uses \"...\".

- The oreo example was broken: the Set-Cookie output doesn't add
  quotes around "doublestuff".

- I had to change the example that prints the class of a Cookie.Cookie
  instance to avoid incorporating an arbitrary object address in the
  test output.

Pretty good score for both doctest and the doc string, I'd say!