Added support for detecting change in objects with relations in nested-dirty.js

[ldeboer]

This update enabled nested-dirty to detect changes of BelongsTo relations. For example, let's say I had an object with a location attribute that links to another model via BelongsTo, and I change the object in the location attribute to a different location object. Before this change would go unnoticed because related objects were skipped over when creating the $cmStatus. However, now, it saves the $pk of related objects to the $cmStatus, so it would now be able to detect that the location value changed (based on looking at the previous vs new location $pk).

[jpulec]

@ldeboer This PR is quite old and out of date. Do you still have any interest in getting it included?