Incorporate addons into EmberApp.

Incorporate the `Project` models, recent addon support into `EmberApp`.

Uses the following hooks/functions that are called on each addon (if the method exists):

* `included` -- Called at the end of the `EmberApp` constructor.
* `treesFor` -- Called at various points in the building up of the final dist tree. Receives the name of the step that is being processed. The return value is expected to be a Broccoli tree.  The following steps are available with this PR:
 * `app`
 * `styles`
 * `vendor`

----

For a sample addon please take a look at: [ember-cli-pretender](https://github.com/rjackson/ember-cli-pretender/blob/master/index.js)

`ember-cli-pretender` can be used (with these changes) via the following steps:

```bash
npm install -g ember-cli
ember new some-awesome-app
cd some-awesome-app
npm install --save-dev ember-cli-pretender
ember server
```

No modifications are needed to the `Brocfile.js` at all.
lets make this mandatory.  Its easy enough for people to write noop functions if they do not implement. But if they typo, we can easily validate and provide very quick feedback.