allow calling httpLog with IncomingMessage obtained from HttpClient

Simple addition to allow constructing an `HttpLog` from the response from a http request. 

Sample usage: 

```javascript
request('http://some-url', function (err, response, body)
{
  var hlog = httpLog(err);
  if (hlog.failed) { ... }

  hlog = httpLog(response);
  if (hlog.failed) { ... }

  // all good
});
```