Add :raw_data option to streaming client.

Added :raw_data option to streaming client. This allows code to do its own JSON parsing or skip parsing altogether.

While using the streaming client today I noticed that I would get "unexpected token" errors. I found that skipping the `JSON.parse` step within the Twitter code and using `Yajl::Encoder.encode` in my code handles the data coming back from Twitter. I suspect it has something to do with UTF-8 but did not investigate further.

`ruby 2.0.0p247 (2013-06-27 revision 41674) [x86_64-linux]`

Thanks for a very useful gem!