square/okhttp — pull request #761 by adriancole (2014-04-25 04:15:13+00:00)
Title: Introduce Call for async invocations.

Breaking out preparation from invocation gives us the ability to cancel an asynchronous request, and in the future expose things like metrics.

```java
Call call = client.call(request); // nothing, yet.
call.execute(callback); // off it goes.
call.cancel(); // whoops..
```