delivery: implemented Client.ListDeliveries

[odeke-em]

Updates #32

• Implemented Client.ListDeliveries.
• Introduced scope oauth2.ScopeDelivery.

NB:

• ListDeliveries hits the /v1 API instead of
  /v1.2

Sample usage:

func main() {
	client, err := uber.NewClientFromOAuth2File(os.ExpandEnv("$HOME/.uber/credentials.json"))
	if err != nil {
		log.Fatal(err)
	}

	delivRes, err := client.ListDeliveries(&uber.DeliveryListRequest{
		Status:      uber.StatusCompleted,
		StartOffset: 20,
	})
	if err != nil {
		log.Fatal(err)
	}

	itemCount := uint64(0)
	for page := range delivRes.Pages {
		if page.Err != nil {
			fmt.Printf("Page #%d err: %v", page.PageNumber, page.Err)
		}
		for i, delivery := range page.Deliveries {
			fmt.Printf("\t(%d): %#v\n", i, delivery)
			itemCount += 1
		}
		if itemCount >= 10 {
			delivRes.Cancel()
		}
	}
}