Update copyright information

* Name "cgit Development Team" as copyright holder to avoid listing
  every single developer.

* Update copyright ranges.

Signed-off-by: Lukas Fleischer <cgit@crytocrack.de>
Add AUTHORS file

Contains a list of contributors with more than 20 patches, to be updated
regularly.

Signed-off-by: Lukas Fleischer <cgit@crytocrack.de>