* compile _document using webpack
* don't emit the bundle file of _document.js
* exclude _document.js from minChunks of CommonsChunkPlugin
* handle creation/removal of pages/_document.js
* improve path handlings
* Immplement the initial singleton Router.
* Use the new SingletonRouter for HMR error handling.
* Use SingletonRouter inside the Link.
* Create an example app using the Router.
* Make the url parameter optional in Router.push and Router.replace
* Add a section about next/router in the README.
