Use resource-based stubs which are automatically namespaced

Was working on the docs for the `ember generate api-stub` command and though it could be nice to actually do this resource-based instead of route-based. This is a first version but it sould basically make the code out-of-the box compatible with ember-data..

This means `ember generate api-stub posts` will generate `server/routes/posts.js` which contains:

```javascript
module.exports = function(app) {
  var postsRouter = express.Router();
  postsRouter.get('/', function(req, res) {
    res.send({posts:[]});
  });
  app.use('/api/posts', postsRouter);
};
```

An alternative would be to use `ember generate api-stub --resource <resource_name>` to override the current behavior.