A suggestion about adding go modules system to the project in order to better manage dependencies and IDEs. While creating a pull request I found difficult to test the change because there was no module file.

I was taking another look at the code and saw that the client had a lot of responsibility like placing orders and listing products. So I created this pull request to discuss the possibility of letting the client having an order interface(maybe) and it accessing it to create orders, or get a specific order.

`type Orders interface {
PlaceOrder(timestamp, signature string, req *http.Request)
[...]
}

type Client struct {
Orders Orders
[...]
}
`

In this example, we would create an order interface with an underlying concrete type, but the client would just use the order interface so we can have a better dependency between the client and the orders and less responsibility in the client.