Add functions to modify cookies to response class

Currently there is no AppFramework way to modify cookies, which makes it unusable for quite some use-cases or results in untestable code.

This PR adds some basic functionalities to add and invalidate cookies.

Usage:
```php
$response = new TemplateResponse(...);
$response->addCookie('foo', 'bar');
$response->invalidateCookie('foo');
$response->addCookie('bar', 'foo', new \DateTime('2015-01-01 00:00'));
```

Existing cookies can be accessed with the AppFramework using `$this->request->getCookie($name)`.