RFC license change to Revised BSD License

Provided that all authors of the Flask-Menu 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:

- [ ] Author: Jiri Kuncar <jiri.kuncar@gmail.com>
- [ ] Author: Krzysztof Lis <krzysztof.lis@cern.ch>
- [ ] Author: Lars Holm Nielsen <lars.holm.nielsen@cern.ch>
- [ ] Author: Matthew Dillon <mrdillon@alaska.edu> (@thermokarst)
- [ ] Author: Tibor Simko <tibor.simko@cern.ch>

Can we please all explicitly express our agreement/disagreement in the comment section of this pull request?