Exposes middleware and service API directly on registry objects

This will make the service and middleware API more transparent and self-documenting, albeit slightly more verbose. As both services and middleware use the registry object internally, they can expose a uniform API, which should lower any learning curve.

The existing API:

```javascript
app.bind("channel", Service);
app.unbind("channel", Service);

app.use("channel", Middleware);
app.unuse("channel", Middleware);
```

The new API:

```javascript
app.services.bind("channel", Service);
app.services.unbind("channel", Service);

app.middleware.bind("channel", Middleware);
app.middleware.unbind("channel", Middleware);
```

As we are still on the 0.x unstable release path, there is no deprecation planned. The current API is removed.