Add ember-export-application-global to default app blueprint.

From https://github.com/stefanpenner/ember-cli/pull/2183#issuecomment-58502411:

> > The addon solves my problem, but it also requires my knowledge of the addon's existence which seems like a lot to ask of new users.
>
> Exactly, I think the cost/benefit trade-off here makes it clear we should include by default