Adds klarna payment provider (#79)

packages/medusa/src/models/order.js

@@ -18,6 +18,7 @@ class OrderModel extends BaseModel {
     // awaiting, captured, refunded
     payment_status: { type: String, default: "awaiting" },
     email: { type: String, required: true },
+    cart_id: { type: String },
     billing_address: { type: AddressSchema, required: true },
     shipping_address: { type: AddressSchema, required: true },
     items: { type: [LineItemSchema], required: true },