node: todos: use gcloud.

Since gcloud-node has changed considerably since #46, I went back over and brushed it up. 

This uses a more Node-friendly structure. There's the app, index.js, which does all the interaction with Datastore. After each action against the dataset is completed (say, adding a todo), it will emit a relevant event ('todo added'). There are listeners for these events in `cli.js`, the "client" which runs the app.

I also threw in a readme to help set some environment variables, so they can run without modifying any source files (though they still have the option, of course).