Release 5.3.1

• Use Webpack to build the server code, allowing webpack loaders like url-loader and css-loader on the server.
• Use Webpack HMR to instantly update server code, instead of restarting the Koa server with piping. Webpack HMR is fast!
• Still keeping the pleasant npm run watch command: the only one needed for server AND client development.
• Moved all Webpack config files to configs/.
• Added a file-loader example that deals with the favicon.
• Upgraded to React Router 2.0.0.
• Updated other dependencies.