Batch request support added

We had to use  [Batch operations of Quickbooks API](https://developer.intuit.com/docs/0025_quickbooksapi/0050_data_services/020_key_concepts/00700_batch_operation). We've created the BatchRequest and BatchResponse models to get this done. The api looks like this

```ruby
    
      batch_req = Quickbooks::Model::BatchRequest.new
      batch_req.add("bId1", Quickbooks::Model::Customer.new, "create")
      batch_req.add("bId2", Quickbooks::Model::Product.new, "create")
      # Add more items to create/update
      batch_service = Quickbooks::Service::Batch.new
      batch_response = batch_service.make_request(batch_req)
      batch_response.response_items.each do |res|
          puts res.bId
          puts res.fault? ? "error" : "success"
      end
```

Could you merge this request or give feedback?

Thanks.