Add ember-cli addon.

Once published to NPM (or just referenced from git) ember-cpm can be consumed by an ember-cli addon with only `npm install --save-dev ember-cpm`.

Since our `src/` directory is native ES6 modules, the following works as expected (from within an ember-cli app):

```javascript
import ifNull from 'ember-cpm/if-null';
```

For reference: https://github.com/cibernox/ember-cpm-cli-test-app/pull/2