provide an Order model

[ColinW520]

As we begin to build out our integration with Shippo, we would like to be able to create orders the same way we create shipments/transactions.

Looking at https://goshippo.com/docs/reference/rb#orders-create, there is no example showing how to achieve this using this gem.

Desired syntax example (emulating example from Shipments):

order = Shippo::Order.create({
  to_address: {
    city: "San Francisco",
    company: "Shippo",
    country: "US",
    email: "[email protected]",
    name: "Mr Hippo",
    phone: "15553419393",
    state: "CA",
    street1: "215 Clayton St.",
    zip: "94117"
  },
  line_items: [
    {
      quantity: 1,
      sku: "HM-123",
      title: "Hippo Magazines",
      total_price: "0.00",
      currency: "USD",
      weight: "0.40",
      weight_unit: "lb"
    }
  ],
  placed_at: "2016-09-23T01:28:12Z",
  order_number: "#1068",
  order_status: "PAID"
})

The changes needed look pretty straightforward given the implementation you have here. I would be happy to submit a PR for this if that would be welcomed.