Show welcome page for route not found handler

[Awilum]

We should have welcome page for route not found handler.
To avoid misunderstanding after Flextype Installation.

/**
 * Override the default Not Found Handler with Flextype Welcome page
 */
unset($app->getContainer()['notFoundHandler']);
$app->getContainer()['notFoundHandler'] = function ($flextype) {
    return function ($request, $response) use ($flextype) {
        $response = new \Slim\Http\Response(404);
        $page = file_get_contents(ROOT_DIR . '/src/flextype/welcome.html');
        return $response->write($page);
    };
};