feat(JS) add webpack build process

[rmosolgo]

Here's my best guess at how to update to React 0.14:

• Write manifests for server/browser, no-addons/with-addons
• Build the manifests with Webpack
• Copy them into the gem source, just like the old React.js files
• Longer discussion: Update to React 0.14 #365

Other changes:

• Server rendering uses ReactDOMServer and react-server.js
• Split the rake task into two (update = build + copy)

Lol:

[rmosolgo]

@zpao Could you take a look at the JavaScript side of this? I think this is an OK way to use the new JS modules in the browser & server, but I'm a bit out of my element!

[rmosolgo]

looks good in my own app, here goes!

[zpao]

Sorry for the delay, this looks good to me!