Include regenerator runtime in addon tree

This PR adds the regenerator runtime in the addon tree. I tried to use `app.import` but not get success due to some errors with glob and absolute paths, so I used the `treeFor` hook. At the moment the regenerator is always included, I think it's ok, because the runtime is small and all will works. I created this [repo](https://github.com/marcioj/esnext-test) if someone wants to test these changes, the [users/index route](https://github.com/marcioj/esnext-test/blob/master/app/routes/users/index.js#L11) is where the async await is used. 

Fixes #3
I tried

```js
  treeFor: function(type) {
    if (type === 'addon') {
      return path.dirname(Regenerator.runtime.path)
    }
  }
```

but don't worked. I receive errors about regeneratorRuntime being undefined.
Am I missing something?