Add AUTHORS file and remade source header

[EPuncker]

Pull Request Prelude

• I have followed proper The Forgotten Server code styling.
• I have read and understood the contribution guidelines before making this PR.
• I am aware that this PR may be closed if the above-mentioned criteria are not fulfilled.

Changes Proposed

Added AUTHORS file and remade source files header.

[Erza]

How did u even generate that list? Seems like it's pretty shit, a few discrepancies I noticed:

• Duplicates of people, even accounts with the same email (Sam <sum9900@web.de>, TheSumm <sum9900@web.de>)
• Ancient contributors who barely ever made any commits, few examples:
  • https://github.com/otland/forgottenserver/commits?author=Tovar
  • https://github.com/otland/forgottenserver/commits?author=Techrlz
  • https://github.com/otland/forgottenserver/commits?author=Skillert
  • https://github.com/otland/forgottenserver/commits?author=paulosalvatore
• Recent contributors missing, few examples:
  • https://github.com/otland/forgottenserver/commits?author=MillhioreBT
  • https://github.com/otland/forgottenserver/commits?author=yamaken93
  • https://github.com/otland/forgottenserver/commits?author=Erza
  • https://github.com/otland/forgottenserver/commits?author=zbizu
• Literally contains Github itself (GitHub <noreply@github.com>)