Online authentication integration

[IotaBread]

Removes the "skip online auth." feature and introduces two replacements. Closes #14

(I'm going to refer to players with a valid license & account for Minecraft as 'official' and as 'unofficial' to those that don't have a license or account.)

• Forced online auth. - Forces all players using an username of an official player to use a Minecraft account. Prevents unofficial players from "stealing" the name of an official player (if the latter hasn't registered in the server).
  • EDIT: Added a config option forced-online-users (an array of usernames) to select which players should use offline auth even if there is an official player with that name.
• Optional online auth. - Lets official players to choose to use online mode authentication with the /onlineauth command, effectively skipping /login authentication.

Note: Both features require the server to use online mode, although I think it could work in offline mode (if the if statements checking the online mode are removed from the code).
Note: Both features give official players their UUID given by Mojang, allowing them to change their account username without losing data (in optional online auth. they have to run /onlineauth again). Unofficial players keep their offline-mode UUID.

I don't know Russian, so I left the added messages in English

[IotaBread]

Apparently it doesn't work outside dev. Gonna fix it tomorrow

[ghost]

I've tested all possible situations that can happen like stolen name conflict, original name conflict, attaching original name after registering non-original name etc. and everything works.. great mod improvement

[Dqu1J]

Will test it later, thanks for contributing!

[Dqu1J]

Please, see my message #14