Oxford: fix for unpacking method

* Fixes checking if variable exist in the oup_packege object.
ftp_utils: download directory fix

* moving to the corresponding ftp directory before downloading
  the files gets rid of the need to manage two different file
  names.

Signed-off-by: Martin Vesper <martin.vesper@cern.ch>
springer_crawler: fix bad print statement
springer_crawler: nested content support

* Adds support for content spread over several pages.

Signed-off-by: fschwenn <florian.schwennsen@desy.de>
Merge https://github.com/inspirehep/harvesting-kit into springer