add http timeout for httpbroadcast

While pulling task bytecode from HttpBroadcast server, there's no timeout value set. This may cause spark executor code hang and other task in the same executor process wait for the lock. I have encountered the issue in my cluster. Here's the stacktrace I captured  : https://gist.github.com/haitaoyao/7655830

So add a time out value to ensure the task fail fast.