Add support for broccoli-jstransform

This PR adds the ability to transpile ES6 to ES5 using [broccoli-jstransform](https://github.com/aexmachina/broccoli-jstransform).

It currently depends on a `jsOptions` option to `EmberApp()` to specify the `ignoredFiles` option so that the contents of `vendor/` aren't processed. We all agree that this approach for providing options to preprocessors isn't a great solution, see [discussion of alternatives](https://github.com/stefanpenner/ember-cli/pull/716#issuecomment-43610558) on this PR (that adds support for providing options to CSS preprocessors).

I don't expect this PR to be accepted for the above reason but thought that it might be valuable for discussion or for anyone else looking to use ES6 with ember-cli.