index: add node.js section and express gcp backend

Front page link for #1050

Blocking on https://github.com/GoogleCloudPlatform/gcloud-node-todos/pull/19 and a deploy on gcloud-todos.appspot.com

/cc @viviancromwell @stephenplusplus @s3ththompson 
```diff
-<a href="http://gcloud-todos.appspot.com" data-source="http://googlecloudplatform.github.io/gcloud-node/" data-content="An Express backend implementation for the AngularJS frontend using Google Cloud Client Library for Node.js .">
+<a href="http://gcloud-todos.appspot.com" data-source="http://googlecloudplatform.github.io/gcloud-node/" data-content="An Express backend implementation for the AngularJS front end using Google Cloud Client Library for Node.js.">
```