harvesting-kit: Filters empty subfields of field 773

* Empty subfields are now removed before being added to the record

Signed-off-by: Martin Vesper <martin.vesper@cern.ch>
Merge pull request #77 from mvesper/filter_field_773

harvesting-kit: Filters empty subfields of field 773
Merge pull request #78 from Dziolas/master

Elsevier: automatic update of delivery dates for packages.
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.