RFC license change to Revised BSD License

Provided that all authors of the setuptools-bower package 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 FOSS community and is similar to the licensing change in our Flask extension packages.

According to the git repository log, here is the full list of authors and contributors:

- [x] Author: Anthony Sottile <asottile@umich.edu> (@asottile)
- [x] Author: Jan Aage Lavik <jan.age.lavik@cern.ch> (@jalavik)
- [x] Author: Jiri Kuncar <jiri.kuncar@cern.ch> (@jirikuncar)
- [x] Author: Lars Holm Nielsen <lars.holm.nielsen@cern.ch> (@lnielsen-cern)
- [x] Author: Tibor Simko <tibor.simko@cern.ch> (@tiborsimko)

Can we please all explicitly express our agreement/disagreement in the comment section of this pull request?