[BREAKING] Implicit response rendering

BaseHttpResponse->display() has been removed!

Now there's default template file name that corresponds to the current
handler class name (e.g. \Some\Namespace\Handlers\ProfileHandler =>
Profile.twig).

You can overwrite it in your handler with
$this->response->set_template_name('OtherTemplate');.

The recommended way to render your handler is to let the PHPFramework do
it. Just assign your variables to the response in your Handler. After
executing your Handler the Framework will render the response.
You can still render it yourself with $this->response->render() and
return the result. Once your Handler returns something the Framework will
not execute the response.