restructured last chapters

chapter15/ch15-express.js

@@ -0,0 +1,10 @@
+const express = require('express');
+const app = express();
+
+app.get('/', (request, response) => {
+  response.send('Hello Express!');
+});
+
+app.listen(3000, () => {
+  console.log('Express app at http://localhost:3000');
+});