Support a new option for returning raw predictions via socket

We have a use case where we want to know the "confidence level" of a prediction, which means we need to look at the raw predictions output of vw.  However, only the final prediction is sent back to the socket connection when running vw in daemon mode.

This pull request adds support for a new command line option, --return_raw, which makes vw return the raw predictions, in place of the final ones, to the socket.  You can also still specify output files for both prediction types.

We thought this might be useful to others as well.

Edit: as a further point of reference, this discussion is very similar to our use case: http://fastml.com/predicting-closed-questions-on-stack-overflow/