BibEdit: fix perform_doi_search function
- Adds 'User-Agent' to the headers send within the perform_doi_search function
- Adds CFG_DOI_USER_AGENT to the bibedit_config file
- Adds CookieJar to store cookies on the pages redirected from the dx.doi.org
- Adds unittests for the perform_doi_search function
Reviewed-by: Javier Martin Montull <jmartinm@cern.ch>