#20 Expiry of user accounts

[James Dominy]

Allow $_CONF to set options that
- specify accounts expire (add expiry date to TUser)
- specify accounts timeout (if last logon more than $_CONF["account_timeout"] seconds ago, account is deactivated, if email being used, send warning to user, if commands being used too, send warning with link to reactivate, and create purge cron to clean out timed out accounts