add populate option

Example usage:
```
User.list({find:{country: "UK"}, sort: "-startTime", start: 10, limit: 20, populate: 'address'})
```

Ideally it would be nicer to have possibility to specify multiple populate fields (http://mongoosejs.com/docs/api.html#document_Document-populate).

E.x. `populate: ['address', 'bankDetais'] `