Added Player:GetUserGroup(), tidied up docs in player_auth.lua
Move ply.IsFullyAuthed to where it sets usergroups. Prevents ply:Is/GetUserGroup() returning a bad result, also gives admin mods more flexability if they want to deal with non authenticated players differently.
Added util.GetUserGroups() - Gets SteamIDs from users.txt table.
Merge pull request #549 from HandsomeMatt/player_auth

Added Player:GetUserGroup(), tidied up docs in player_auth.lua