springer_crawler: nested content support

* Adds support for content spread over several pages.

Signed-off-by: fschwenn <florian.schwennsen@desy.de>
Merge pull request #57 from fschwenn/springer2

springer_crawler: nested content support
edpsciences_package: Astrophysics subject added

Signed-off-by: Georgios Papoutsakis <georgios.papoutsakis@cern.ch>
global: refextract indentifier added

* Adds the subfield 9995C9:refextract for
  references extracted with refextract.

Signed-off-by: Georgios Papoutsakis <georgios.papoutsakis@cern.ch>
global: Bibrecord dependecy removed.

Signed-off-by: Georgios Papoutsakis <georgios.papoutsakis@cern.ch>
edpsciences_package: attach fulltext fix

* Modifies _attach_fulltext function to handle url timeout errors
  and retries to open the url 10 times.

Signed-off-by: Georgios Papoutsakis <georgios.papoutsakis@cern.ch>
jats_package: empty affiliation fix

* Handles the case of empty affiliations.

Signed-off-by: Georgios Papoutsakis <georgios.papoutsakis@cern.ch>