Update nodejs demos

I've taken the liberty of trying to update the nodejs demos in this repo to work with the new google-api-nodejs-client. I haven't actually tested the changes so if someone could give this a second look & perhaps test it, that'd be awesome.

Plus general feedback is always welcome. Maybe we want to scrap this altogether and create a better demo?
I mean the `.json` based credentials instead of `.p12` files.

My understanding is that the former doesn't need you to specific the service account name since both the certificate and the identity are present in the file.