Added support for using DateTime objects in cookies

As previously discussed: https://github.com/chriso/klein.php/pull/213#issuecomment-49532907
ResponseCookieDataCollection implements `IteratorAggregate`, `ArrayAccess`, `Countable` and makes it an array of ResponseCookie.

![image](https://cloud.githubusercontent.com/assets/671923/4752631/d81c2a98-5aad-11e4-8997-6e0b92ddd42f.png)

vs

![image](https://cloud.githubusercontent.com/assets/671923/4752640/fa874d24-5aad-11e4-831c-3028b771d07d.png)