Update npm start script to run webpack before the server

Also, update `.gitignore` to ignore the bundle output

Author: remarkablemark

.gitignore

@@ -1,3 +1,4 @@
 node_modules
 *.swp
 *.log
+public/bundle.js

package.json

@@ -4,7 +4,7 @@
   "description": "A tutorial of building a universal web app with React.",
   "author": "Mark <[email protected]>",
   "scripts": {
-    "start": "node server.js"
+    "start": "webpack && node server.js"
   },
   "main": "server.js",
   "keywords": [