Implement Readable stream for runQuery in datastore

If no callback is given to runQuery, imply a Readable stream mode, e.g.:

``` javascript
var q = dataset.query(/*…*/);
dataset.runQuery(q)
  .on(“data”, function (entity) { console.log('I got one entity:' + entity); })
  .on(“end”, function (entity) { console.log('This is the end, my only friend'); })
```

And handle providing results as long as they are available.
