Enable properly nested addons

See #2354 for context.

This adds a `parentAddon` context for any nested addons included in a project. The `parentAddon` is supplied to each child addon on the addon instance as well as passed into the `included` hook.

The child addon is then free to determine if it should modify the `parentAddon` or the `app` from within the `included` hook.

This enables addons to have their own isolated dependencies which don't interfere with the root app's preprocessors. I.e. I can write my addon in Coffeescript, and the parent app doesn't need to depend on the Coffeescript addon directly.
Happy to throw one in.

I didn't include it out of the box, because the fixture's `app/routes/application.js` imports the result of the preprocessor, so essentially acts as the assertion and fails when the preprocessor fails to run.