Changed plugin name from "Invitation[0x20]Only" to "InvitationOnly"
because the use of the space-character (0x20) in its name is discouraged
by bukkit.
Added some comments
Added ability to save the name of a player in the users.yml and update
it on the (atempted) join of an invited player or member.
Added import of whitelisted players as members when there is no
users.yml (aka on the first plugin start)