Import / Download / Delete working
All bug should be fixed for the import / download / delete functions.

* Some problems with utf-8 and homogeneize_latex_encoding in
python-bbtexparser are bypassed and will be cleaned in a better way when
the latest version will be available in pip.
* Tweaked regex for isbn, which was not cas insensitive and forgot about
spaces separated numbers.
* File entry in arXiv bibtex is now deleted to avoid confusion.