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 
Good point, I didn't even think of this problem. I suppose we will need some sort of standard to section these off.

One thing is, I don't think backend implementations should be "Something + Angular" -- a backend should serve all of the examples, not just one. It's early on, so Angular is the only example completed, but that won't always be the case.

Maybe something like:

Node.js > Express + gcloud-node, or
Node.js > Express + Google Cloud Platform

// @addyosmani