Single Middleware type implementation

Changed Route.php to support the application middleware
(\Slim\Middleware instances).
This way:
- 1 single middleware concept and implementation
- "route middleware" can be executed also after the route callback

$route = $s->get('/bar', $mw1, $mw2, $callable);
still works but requires that $mw1 and $mw2 are instances of
\Slim\Middleware.