Added a script to be run as a cron job to generate periodic notifications,
and also two script for controlling the system.

Installation will copy tea-break.sh to /etc/cron.daily, resulting in that
script being run each hour by anacron.

disable-alarm and enable-alarm should be placed on the path, such as
/usr/local/bin for installing outside the package manager. The cron job
checks to see if notifications are enabled by checking the existence of
a particular file. These two scripts enable and disable notifications by
respectively creating and removing that file.