Added support for using DateTime objects in cookies

As previously discussed: https://github.com/chriso/klein.php/pull/213#issuecomment-49532907
Its not a bug with PhpStorm, and if you want it to be fixed, post an issue there: https://youtrack.jetbrains.com/issues/WI

Meanwhile, ArrayCollection's of Entities should be type-hinted. Thats that.