Fix in mongo model.getUser to only return the _id. Prevents full user ob...

...ject from being assigned to userId parameter in save methods.
Please could this be a single two space indentation as in the rest of the file.

Also, for brevity, could this be either:
```js
if (err) return callback(err);
callback(null, user._id);
```
or
```js
calback(err, user && user._id);
```