Allow node references to import and export

We recently came across a situation where we wanted object references in salesforce to map to object references in drupal. So, we added node and user reference support to the sf_node module.

Example:
Courses in Salesforce map to Course nodes in Drupal.
Sections in Salesforce map to Sections in Drupal.

In salesforce, each section belongs to a course and has a reference to it. So, we would first import the course object. Then when we import the section object, it can look up which course node it should be referencing.

I realize that no new features are being added to the before the release of the stable version of this module, so I'm just throwing this out there in case other people need to do the same thing.