Added support for using DateTime objects in cookies

As previously discussed: https://github.com/chriso/klein.php/pull/213#issuecomment-49532907
This open source project, used by users of many IDE's, will not cater to a specific IDE's non-standard styling.
Telling me what the code-style of the project "should be" during a feature PR will get you no where and will do nothing but annoy me.

I'm not even asking you to press a space-bar a bunch of times, I'm asking you to **not** frivolously change the documentation style that already existed.

I will not merge these changes in unless the project's style is kept intact. This project, and its potential consistency, is bigger than just one contributor or one IDE's opinions.