Add purchased products to response in purchase

## 概要
購入時のレスポンスに商品情報を追加しました

## レスポンス例
```
{
  "purchased_products": [
    {
      "barcode_id": 4903326112852,
      "category": 0,
      "id": 1,
      "price": 108,
      "stock": 12,
      "store_id": 1,
      "product_id": 1,
      "created_at": "2014-09-19T17:07:28.000+09:00",
      "updated_at": "2014-09-19T17:07:28.000+09:00",
      "name": "サクサクしっとりチョコ"
    },
    {
      "barcode_id": 4903326112853,
      "category": 0,
      "id": 2,
      "price": 208,
      "stock":  20,
      "store_id": 1,
      "product_id": 2,
      "created_at": "2014-09-19T17:07:28.000+09:00",
      "updated_at": "2014-09-19T17:07:28.000+09:00",
      "name": "ぷりぷり鮭おにぎり"
    }
  ],
  "response_from_webpay": true,
  "response_from_pos": true
}
```