Added support for using DateTime objects in cookies

As previously discussed: https://github.com/chriso/klein.php/pull/213#issuecomment-49532907
This will be done automatically, in that order, by **your** IDE.
[`throws` tags are not after in this library's code base](https://github.com/chriso/klein.php/search?utf8=%E2%9C%93&q=throws). Please follow suit with your deprecated tag.