A fix for the ModelResource when it contains more than one m2m field
pointing at the same sub-resource.  Attempts to create a m2m relationship
during a PUT or POST when multiple m2m fields were pointing at the same
sub-ModelResource would result in only one m2m successfully creating the
relationship.

 - Added an optional uniqueness to the ModelResource.create_identifier()
 via a field_name argument

 - update the ModelResource.save_related() and ModelResource.save_m2m()
 to make use of the optional field