RFC license change to Revised BSD License

Provided that all authors of the Flask-SSO extension agree, this PR would change its license from the GNU GPLv2 to the Revised BSD License. The change to a more permissive free software license is motivated by easier collaboration within the Flask community.

According to the git repository log, here is the full list of authors and contributors:

* [x] Author: iason <json.andr@gmail.com> (@Json-Andriopoulos)
* [x] Author: Jiri Kuncar <jiri.kuncar@cern.ch>
* [x] Author: ThiefMaster <adrian@planetcoding.net> (@ThiefMaster)
* [x] Author: Tibor Simko <tibor.simko@cern.ch>

Can we please all explicitly express our agreement/disagreement in the comment section of this pull request?