Relicense the code under Apache-2.0

The old gpl license might make us vulnerable to oudside contributers, as
discussed in the tech share meeting. Let's relicense under Apache-2.0 to
also address some potential patent related issues.

Relicensing requires the consent of all copyright holders. I’ve tracked
down all contributions, and they were all done by people working at
Nobleo. This means Nobleo is the sole copyright holder.