Added install_launchd.sh
This script generates and loads launchd plist which will execute a specified script once a week.
The plist will be written to /Library/LaunchAgents/org.macports.stats.plist
The date and time the script will run is determined as follows:
Day of the week is the machine's hardware UUID mod 7
Hour and minute is set to the time that install_launchd is executed
