- Add a test for :max-age and :expires to (wrap-cookies ...). There were
  no real tests to exercise the base input cases (int, string) which the
  comment block states. This test just makes sure it fulfils that
  contract.
- clj-time is now a project dependency
- :max-age accepts an Interval as input. Updated (write-attr-map ...) to
  accept an Interval (from JodaTime) as well as an int. The interface
  from clj-time is used.
- :expires accepts a DateTime object. It converts the DateTime object in
  the equivalent RFC822 which the cookie spec requires.
- Added pre-conditions for :max-age and :expires to make sure that they
  only accept Interval and DateTime, respectively.