create fragment arrays using the store API

I know you can use the store API to create fragments e.g.

```
person.set('name', store.createFragment('name', {
first : 'Hugor',
last : 'Hill'
}));
```

but is there an API to do the same with fragmentArrays?

Start with the store API and the createFragment example in the issue. Trace the existing fragment creation entry point and compare it with fragmentArray handling; done means the repository provides or clearly documents a way to create fragmentArrays through the store API.

索引モデルが issue の本文から書いたものです。