BibRank: stop on citation losses

* When bibrank -w citation fails because we lost too many citations,
  displays in the log for each record how many citations they lost.
* Abort bibrank task when the loss for a record is greater than x.
  (x is set in the citation.cfg)
BibRank: wrong diffs when checking citation losses

* The differ was using the cites dictionary twice when checking for
  the overall citation balances in bibrank -w citation.