Enhanced biblatex

Two issues:
1.   The BibLaTeX translator have been included in the official repository with a different id, but my  translator with the old id is still available here and might be interesting to some people since it contains a few extra "features":
https://github.com/andersjohansson/zotero-biblatex-translator

    But if people use that one they can customize zotelo-translators to add it.
2.  BibLaTeX recommends `\addbibresource{}` instead of `\bibliography{}`, but then complete filenames including extension must be given, which means zotelo would write "bibliography.bib.bib". I added a small check to fix this.