BibRank: check citation indexing necessity
BibRank: check citation indexing necessity

* Skip processing references if no record is found on the site with
  defined reference tags.  Useful for sites not having citations
  but keeping citation ranking method from the demo site.

* Using register_exception() now to signal exceptional situations
  e.g. if necessary tags are missing in the config file.

inveniosoftware/invenio,tiborsimko/invenio,inveniosoftware/invenio,tiborsimko/invenio