Implement middleware for responders / subscribers

Certain functions such as auth or request decoration would make use of middlewares, as in, virtually everywhere (Express & co).

The following code...

``` js
responder.on('request', function(req, cb) {
// do something;
});
```

should still work with a middleware:

``` js
responder.on('request', middleware, function(req, cb) {
// do something if middleware allows
});
```