authentication. utility to generate a unique user ID from a username.
Merge branch 'config' into multiuser
mail store. mailbox key-value store name is changed.

msg -> message
mbox_# -> mailbox_#
server configuration class. path utility for multi-user key-value store.
mailbox data structure is changed from single user to multi-user.
constant variable of mailbox data structure version
server internal user identity is changed from username to unique user ID.
at server internal, parameter to open key-value store is changed from file path data to user identity data.
server configuration class. path utility to make directory should return the flag with which a directory was made.
path utility returns the path string that was made.
if the directory already exists, path utility returns a nil.
logging of making a parent directory of key-value store of mailbox for a user.
changelog: * lib/rims/mail_store.rb, lib/rims/protocol.rb, lib/rims/server.rb, lib/rims/version.rb: multi-user mailbox. layout of parent directory of mailbox key-value store is changed from single user layout.
Merge branch 'multiuser'