Remove app.lookup and make the functionality available as app.service

It turned out that `app.lookup` is somewhat redundant and not very descriptive. Instead I think it is a lot more explicit to make service lookup available as `app.service(path)`:

```js
app.service('todos').create({ text: 'Do something!' }, {}, function(error, data) {
  console.log('Created Todo', data);
});
```

To maintain backwards compatibility simply assign `app.service` as `app.lookup` like so:

```js
app.configure(function() {
  this.lookup = this.service;
});
```