Migrate from MARC to JSON format

• Stores inspirehep.net and cds.cern.ch can return a JSON string instead of an XML one.
• For example: http://inspirehep.net/record/1471725?of=recjson
• More on the JSON API: https://inspirehep.net/info/hep/api?ln=fr
• Map JSON / MARC: https://github.com/inspirehep/invenio/blob/prod/modules/bibfield/etc/atlantis.cfg
• Comparison between JSON and MARC in CodesJsonMarc.pdf and Json_fields.pdf