Normalize offline player's name before creating an UUID

This is a somewhat breaking change, so I will understand if you don't want to get this pulled.

This converts player's name to lower case when generating their UUID if the server is in offline mode, so "User", "uSer", "user"... all have the same UUID.

This matches not only the behaviour of most plugins, which ignore the casing, but also the Minecraft login system before UUIDs were introduced, where the case didn't matter either.

Right now banning an user is hard, as they can just change the case, therefore having the same nick to most plugins, yet still be able to bypass the UUID ban.