Add Flusher to the ResponseWriter wrapper

I was working on an app that required Server Sent Events (http://en.wikipedia.org/wiki/Server-sent_events).
Using Martini, I was able to piece together the basics fairly quickly, but noticed that my client wasn't getting all the data sent from the server.  The culprit was that the ResponseWriter wrapper in Martini didn't support the Flusher interface.  

It's a fairly simple interface to implement, only requiring one method, so I did so and present it in this pull request.  
I chose to approach it the same way the Hijack function reaches into the default net/http Hijacker