Bibcheck: Adds check_arxiv_category implementation

* Adds the check_arxiv_category bibcheck plugin to check if a certain
  record is in the hep category on arXiv

* Adds the arXiv id to the record in the case that it doesn't exist
  yet, but can be found on inspire using the doi

* If none of the above works, the record is not changed

Signed-off-by: Martin Vesper <martin.vesper@cern.ch>
Merge pull request #28 from mvesper/arxiv_category

Bibcheck: Adds check_arxiv_category implementation