Added support for using DateTime objects in cookies

As previously discussed: https://github.com/chriso/klein.php/pull/213#issuecomment-49532907
This doc-block should have a description (preferably the one that already existed)
It should also not remove the `@access` tag.
The return statement shouldn't use `$this`, as its not part of the current style
And the deprecated tag should be the first tag.