Added support for using DateTime objects in cookies

As previously discussed: https://github.com/chriso/klein.php/pull/213#issuecomment-49532907
We'll want to allow null here, so it should be:

```php
if (null === $this->expiration) {
    return null;
}

return $this->expiration->getTimestamp();
```

Also, a new line should be used to separate the end of the `trigger_error()` call and the return statement.