Adding automatic installer and update check

Thanks for writing this beautifully simple agent!  I've been looking for ways to reliably get my public IP, and you have by far the shortest version I've seen so far.  Well done!

As much as I love editing configuration files by hand, I prefer to rely on my tools to ensure I don't screw up :)  The ./install script will now automatically install the plist file in ~/Library/LaunchAgents with all parameters changed, and it even ensures the log file is writeable.

Thanks!