FEATURE: Export multiple records (as BibTeX).

Closes #132.
FIX: BibTeX exports are too generic.
TEST: Report the activity of a researcher since a given year as a bibtex file
MERGE: feature #132 branch.

- Fixed the related test so that it waits for the download.
- Fixed the feature so that the filename does not contain spaces.
FIX: Indentation of BibTeX exports.
FIX: French localization for "issue number".

See https://github.com/benel/Tire-a-part/wiki/Numérotation-des-périodiques