bug fix for --stream + mitmproxy, stream documentation

See https://github.com/mitmproxy/mitmproxy/issues/88

5bb4e37 is a fix to handle the fact that when using mitmproxy with --stream it would get an exception saying that there was no f.response.reply.  I'm not entirely sure what this is all about, but putting in the appropriate check to see if f.response.reply exists seemed to solve the problem.

5bb4e37 is some basic documentation for the streaming functionality.