RFC license change to Revised BSD License

Provided that all authors of the Flask-Registry 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: Jiri Kuncar <jiri.kuncar@cern.ch>
* [x] Author: Lars Holm Nielsen <lars.holm.nielsen@cern.ch>
* [x] Author: Tibor Simko <tibor.simko@cern.ch>
* [x] Author: Yoan Blanc <yoan@dosimple.ch>
* [x] Co-authored-by: Adrian-Tudor Panescu <adrian.tudor.panescu@cern.ch> (@adrianp)
* [x] Co-authored-by: Esteban J. G. Gabancho <esteban.jose.garcia.gabancho@cern.ch> (@egabancho)

Can we please all explicitly express our agreement/disagreement in the comment section of this pull request?