elsevier_package: various fixes

* Encodes unicode objects to str.

* Fixes imports.

* Fixes on reference harvesting.

* Fixes error handling.

Signed-off by: Georgios Papoutsakis <georgios.papoutsakis@cern.ch>
Merge pull request #14 from GiorgosPa/elsevier

elsevier_package: encodes unicode objects to str