plotextractor: improves PDF harvesting from arXiv
plotextractor: improves PDF harvesting from arXiv

* Changes from urllib to urllib2 when downloading PDFs in order
  to take advantage of better error handling upon non-successful
  download.

* Adds suffix '.pdf' to all PDF download URLs to arXiv to avoid
  the internal arXiv redirect from 'arXivID' to 'arXivID.pdf'.

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