The GeocoderAPI is just a web service. Use RequestBuilder to build a request along API guidelines and call it. If you want you could also dress it up to make it fancier, i.e. made you own getZip(Lat, Lon) and have that make the request. Of course, keep API rate limits and daily query limits in mind. For this reason, I prefer to do it server side so the repeat requests can be cashed, to stay within free limits.
Sincerely,
Joseph