[WIP] online/offline syncing loses hasMany relationship

Related issue: https://github.com/kurko/ember-sync/issues/15#issuecomment-48251695.

Can't get to meaningful error messages yet, it stalls with:

> Error: Assertion Failed: You looked up the 'cartItems' relationship on '<(subclass of DS.Model):ember311:1>' but some of the associated records were not loaded. Either make sure they are all loaded together with the parent record, or specify that the relationship is async (`DS.hasMany({ async: true })`)
1. Try doing `.push()` instead of `.createRecord()`.
2. If it doesn't work, add it directly to the online database:

```js
fixture = JSON.stringify({
  inventoryItem: {
    records: { // this schema is how LSAdapter works
      2: {
        id: '2',
        name: "Fender",
        description: "Fender 1",
        price: 200.0,
        entry_for_sale_id: 2,
        on_sale: true
      }
    }
  }
});
localStorage.setItem('onlineStore', fixture);
```