add event-emitter support across transport

[tjmehta]

Love substream and primus thanks for the modules!

This is a working attempt of adding event-emitter support to substreams (so that they emit events across the wire)

I am not super knowledgeable w/ substream, so please let me know if there are any side effects. Currently I am ignoring data and end events because they are already handled by substream.

I am also, unsure how you would like error events to be emitted across the wire. I was thinking of using http://npmrepo.com/error-to-json to convert the error to json before sending it over the wire, and then parsing as an error on the receiving end.

Let me know what you think.

Thanks!