Implemented an Admin List

FULL CHANGELOG:

- Admins List implemented
- Added 3 new strings to Common: CLIENT_VERSION, SERVER_VERSION and
ADMIN_MARKER.
- Added a new function on ClientHandler: isAdmin(string playerName).
- The admin marker is not working because I didn't implement it yet.

I had to do it Minecraft style because I tried with storing the player
name and the player Guid, but it didn't work, it said that the player
wasn't an admin.