Use $EDITOR

* Use $EDITOR to edit the bibtex entries, as `git commit` or `crontab -e` does.
* Bugfixes that prevented the import of an article without associated
  bibtex entry or without doi.
Upgrade to latest version of bibtex-parser
Beginning of refactor
Refactor in progress

TODO :
* Use new API from bibtex-parser
* Split addFile / downloadFile in main to a function in main (interface)
and one in backend
Move server to a new branch
Various re.compile
Check output before processing the whole file for ISBN and DOI search
Typo in README
Flake8
Resync function. To be tested…
Update parsed2Bibtex
Forgot to add tools.py file
Unstable indicated in README