global: translation fixes

* Adds configuration file for message extraction.

* Extracts messages from source code files.

* Usage: python setup.py extract_messages -k '_(' \
  -F mapping.cfg -w 100 --input-dirs invenio \
  -o invenio/base/translations/invenio.pot --sort-output

Signed-off-by: Eirini Psallida <eirini.psallida@cern.ch>
Reviewed-by: Jiri Kuncar <jiri.kuncar@cern.ch>