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
No need for `pickFiles` here, just returning the `dirname` variable should do the trick.