Respond to timeouts with HTTP 408

I've updated `proxy_test.go` but note that the test was failing even before I modified it. 

Fixes mailgun/vulcand#39
Can you use a more robust way to detect the timeout error, like this:

```
if netErr, ok := err.(net.Error); ok && netErr.Timeout() {
    p.replyError(errors.FromStatus(http.StatusRequestTimeout), w, r)
}
```