Modular EAK

Hi, 

This is a new branch and is a major change to how the app folder is managed.

This is an attempt to address the problem outlined in https://github.com/stefanpenner/ember-app-kit/issues/302

This pull request is a separate branch. If this looks like the right path, I can rebase to a single commit. But because this touches and moves a lot of files I wanted to submit the PR without quashing so you can see the iterative stages.

Welcome, ideas for optimizing this work flow. I have a few other related PRs sitting in wait if this looks like a good idea.

To see this in action try pulling down the `modular-eak` branch from my fork

https://github.com/eccegordo/ember-app-kit/tree/modular-eak

Reset app folder with
```
grunt purge
```

and then depending on your preferences

```
grunt project:minimum
```
or
```
grunt project:starterkit
```

Welcome feedback.