WIP: Support ES6 module syntax

## Abstract

We can use es6 module syntax on asset pipeline.
This way is the same as [ember-cli](https://github.com/stefanpenner/ember-cli) .

The setting specified ember-rails is the followings:

* `my_app.js.es6`
  ``` javascript
  //= require ember-rails/application
  //= require_self
  //= require_tree

  import App from 'ember-rails/application';

  App.create();
  ```

* `application.js`
  ``` javascript
  // = require ./my_app

  require('my_app');
  ```

## TODO

* [x] Support es6 module syntax
* [ ] Support generator option `javascript-engine=es6`
* [ ] Add test
* [ ] Write how to use es6 module syntax to README