User management

• recorded in the data/users.xml folder
• each new user is required to enter the name, then a new ID is generated based on the name(ex: john doe - jd1 - the first user with the jd initials), plus an unique index id for each user.
• an existing user can log-in using the ID
• all the tappings will be stored in the data directory, in filenames corresponding to the user id, e.g. jd1.xml