#1 Register nick names

[Csaba Markus]

Currently the system does not handle registered nick
names so it is possible to use any nick name (username)
that is not currently logged in.

Make it possible to request the registration of a nick
name. In the beginning, this can be an immediate action
without any email-confirmation but later it can be
added to require a valid e-mail address.

At login, do not let the user in if the nick is
registered and the correct password is not given.

Do not store clear text passwords. Instead, store some
hash of the password (e.g. MD5).