Patch to intergrate with 'entity' and enitity_metadata_wrapper()

Here is a minimal patch to allow fields of this type to integrate with the 'entity' module and specifically the wrapper generated by entity_metadata_wrapper().

It causes this field to be treated like an integer. I think it would be better if it were treated as a node at a specific revision (because than you'd get useful tokens from entity_token as well). But this atleast prevents it from crashing. :-)