Use Project model from EmberApp.

* Add `Project.closestSync` (we cannot do async in `Brocfile.js` right now).
* Use `Project.require` and `Project.name` logic for `EmberApp`.
* Remove boilerplate/defaulted params from `Brocfile.js`.

Fixes #397.

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Making the `EmberApp` use `Project` is the first step to adding centralized logic for discovering addons.