Enable watching the data directory for data updates

If you have a server running `geoip-lite`, and you want to update its geo data
without having to restart it, you can enable the data watcher to automaticaly
refresh geo data upon file changes in the data directory. This tool can be used
with `npm run-script updatedb` to periodically update geo data on a
running server.

This pull request contains:

README.md: update to includes the new watching function
lib/geoip.js: implement the watching functions
lib/fsWatcher.js: generic fs watcher