docs(README): bunch of rewritings

* re-organized auth section
* added basic snippets for datastore and storage
* improved some links
* removed pubsub
Best to show the callback signature. Also should be `dataset.key` (going to fix that in the `datastore.js` jsdoc now):

```js
dataset.get(dataset.key('Product', 'Computer'), function(err, entity) {});
```