Added support for using DateTime objects in cookies

As previously discussed: https://github.com/chriso/klein.php/pull/213#issuecomment-49532907
Nowhere in [PSR-2][psr-2] (or [PSR-1][psr-1] spec that it inherits) does it mention anything about return type documentation.
This is not part of the standard.

[psr-2]: https://github.com/php-fig/fig-standards/blob/master/accepted/PSR-2-coding-style-guide.md
[psr-1]: https://github.com/php-fig/fig-standards/blob/master/accepted/PSR-1-basic-coding-standard.md