Adding mock and mock routes

api-spec/mock.json

+{
+    "carts": [
+	{
+	    "id": "57a98d98e4b00679b4a830af"
+	}
+    ],
+    "items": [
+	{
+	    "id": 1,
+	    "quantity": 10,
+	    "unitPrice": 1.99,
+	    "cartsId": "57a98d98e4b00679b4a830af"
+	}
+    ]
+}

api-spec/routes.json

+{
+    "/carts/:cartsId/items/:id": "/items/:id"
+}