add Card.retrieve(customer_id, card_id) with request from api

omise/__init__.py

@@ -413,6 +413,22 @@ class Card(_MainResource, Base):
         '4242'
     """
 
+    @classmethod
+    def _instance_path(cls, customer_id, card_id):
+        return ('customers', customer_id, 'cards', card_id)
+
+    @classmethod
+    def retrieve(cls, customer_id, card_id):
+        """Retrieve the card details for the given :param:`card_id`.
+
+        :param customer_id: a customer id of card id.
+        :type customer_id: str
+        :param card_id: a card id to retrieve.
+        :type card_id: str
+        :rtype: Card
+        """
+        return _as_object(cls._request('get', cls._instance_path(customer_id, card_id)))
+
     def reload(self):
         """Reload the card details.