Better handling of populated fields

[FloGou]

When an object is saved, we try to update it with the values returned by the query. Unfortunately, this query doesn't offer the opportunity to populate nested objects in the returned object (or so it seems).

Currently, you can pass an array of strings to the save method, and the ORM will GET the object again.

A better solution would be to maintain a _populate array on the object, and implement a refresh method.