title: How to implement an async Callback using Square's Retrofit networking library
body: <p>As an iOS developer beginning to work with Android I came across Retrofit. I understand how to implement synchronous requests but am having trouble implementing asynchronous requests with success/failure callbacks. Specifically, the Callback syntax is unclear to me and there are no concrete examples of how to do this on the Retrofit <a href="http://square.github.io/retrofit/" rel="noreferrer">website</a>, the Square <a href="http://corner.squareup.com/2013/05/retrofit-one-dot-oh.html" rel="noreferrer">blogpost</a> introducing Retrofit, or elsewhere that I've seen. Can someone please post some example code on this? I filed an issue in the Retrofit repo asking that they update the README with this info. </p>