added 'index_fields' option for reference fields.

when a document's toArray method is invoked, the data returned for
references now can be enriched with more information than just id and
module. to have any field of the referenced document to be contained
within the parent array representation add a 'index_fields' options to
the corresponding reference element inside the dat0r module.xml.
then add the 'foreign fields' one per option.