title: Lua socket asynchronous calls
body: <p>I am writing a program that uses Lua socket to communicate with a http server.
  The API that I am using is "socket.http.request", and I have found that it is synchronous. My understanding is that it waits until it gets some response or time outs. 
  Is my understanding correct? If so, I'd prefer to use an asynchronous API.</p>
  
  <p>I also found another API "socket.http.request_cb", which calls a call back function when the request is processed. However, it doesn't seem to work here. (This API is not available on the version that I'm using.) I'm using Lua 5.1 and Lua socket 2.0.2 here. Could anyone let me know which version of Lua or Lua socket has this API?</p>