Added support for using DateTime objects in cookies

As previously discussed: https://github.com/chriso/klein.php/pull/213#issuecomment-49532907
Hmm, we're changing a protected property name here... that could mess up some people that have extended the class and relied on the property name (without using the proper getter and setter methods)... not sure how I feel about this one.

Also, please don't remove the `@access` tag until they're all removed later.