Handle upgrade event to proxy WebSockets

[simshanith]

https://github.com/nodejitsu/node-http-proxy#proxying-websockets
Issue #2

[simshanith]

I can rebase & reopen against the develop branch, if preferred.

[simshanith]

I tested this branch with the CLI against generator-socketio, and it appears to work well.

Steps to reproduce:

• Clone my fork at the PR branch, cd to it.
  • git clone -b ws-upgrade https://git@github.com/simshanith/foxy.git && cd foxy
• Install fork as local global module.
  • npm link
• Install yo and generator-socketio globally
  • npm install --global yo generator-socketio
• Generate the Socket.IO app in a separate folder of your choosing.
  • yo socketio
• Correct the Gruntfile.js, removing the "false" task run attempt. (this may be a bug answering "no" to the "Do you want Bootstrap?" prompt)
• Change the hardcoded Socket.IO client connection in public/js/app.js to be relative to origin:

var socket = io.connect(window.location.origin);

• Build assets & launch the server via default Grunt task.
  • grunt
• Default port for yo-socketio server is 1337. Proxy that with Foxy in another process.
  • foxy http://localhost:1337
• Open network developer tool in browser on the Foxy URL, and make sure no more XHRs are getting triggered after upgrade, and that the WebSocket connection returned a successful 101 Switching Protocols.

After testing, you can npm unlink from the forked foxy directory to uninstall as global Foxy. Note, you will need to reinstall foxy if previously installed.

cd foxy && npm unlink && npm cache clean foxy && npm install --global foxy

[simshanith]

Rebased & force pushed to fix a single-quote instead of double-quote JSLint rule. Appveyor didn't seem to like that.

Anyway my second commit gives a "true positive" from what I can tell, after taking the "false positive" from my first commit to a "true negative" and then back around to "true positive".

Note, however, that dependents such as BrowserSync will need to attach the handler themselves after creating the http(s) server. It should be exposed as app.handleUpgrade.