Modified the scripts which enable and disable alerts so that the users
crontab will be preserved.

enable-alarm will now save the previous crontab in a temp location, and
disable-alarm will restore it. This mechanism will not pick up any changes
that the user has made to their crontab between enabling and disabling
alerts, though.
Added config.sh, which is included by the other shell scripts and defines
configuration constants PREFIX and RUNTIME. This requires changes to the
enable and disable scripts, as well as the installation script, so that
config.sh is available post installation.

Also changed the RUNTIME default to the user's home directory, because the
old version of the crontab would be lost when the computer powers down if
it is stored in /tmp.