Elsevier: Fix for Elsevier doi finding and fixes adding new field.

* Changes search_pattern to perform_request_search in Elsevier to
  better find recid basing on DOI,
* Adds casting values to strings in record_add_field in utils.py.
Merge pull request #80 from Dziolas/master

Elsevier: Fix for Elsevier doi finding and fixes adding new field.