Add invoice model to query invoices

lib/chargify_api_ares/resources/invoice.rb

@@ -0,0 +1,20 @@
+module Chargify
+  class Invoice < Base
+
+    def self.find_by_invoice_id(id)
+      find(:first, params: {id: id})
+    end
+
+    def self.find_by_subscription_id(id)
+      find(:all, params: {subscription_id: id})
+    end
+
+    def self.unpaid_from_subscription(subscription_id)
+      find(:all, params: {subscription_id: subscription_id, state: "unpaid"})
+    end
+
+    def self.unpaid
+      find(:all, params: {state: "unpaid"})
+    end
+  end
+end