Add serve script

Prior to commit 7792fa0 there was a short-hand script to run the live-server, the tutorial also mentions a `npm run serve`.

Author: phoqe

package.json

@@ -30,6 +30,7 @@
     "live-server": "^0.9.2"
   },
   "scripts": {
-    "postinstall": "jspm install"
+    "postinstall": "jspm install",
+    "serve": "live-server ./app"
   }
 }