Added support for using DateTime objects in cookies

As previously discussed: https://github.com/chriso/klein.php/pull/213#issuecomment-49532907
Well I'm sorry but I'm not gonna go press spaces for you every time you want. You're way too stubborn about your coding style. Heres what the project should abide to:

PhpStorm && PSR-2 > PSR-2 > Your coding style