Re-emit 'start', 'forward' and 'end' events in RoutingProxy

Created by: coderarity

Fixes nodejitsu/node-http-proxy#214. This will re-emit more events from RoutingProxy so that you can listen for those events yourself. Be VERY careful for the following case, which will add a new listener for every request:

//DO NOT USE THIS CODE! IT'S BROKEN!
var httpProxy = require('http-proxy');

httpProxy.createServer(function (req, res, proxy) {

    var buffer = httpProxy.buffer(req);

    proxy.on('end', function() {
      console.log("The request was proxied (server)");
    });

    proxy.proxyRequest(req, res, {
      host: '127.0.0.1',
      port: 8080,
      buffer: buffer
    });

}).listen(8000);

It would be good if we could fix this case! This is a very easy problem to run into!