Changes:
* Split `Header` component trough `_document.js` and `_app.js`
* Attached Router events with a way described in the [documentation](https://github.com/zeit/next.js#router-events) (though in the documentation is says it should be `Router.events.on` while I managed to get it working only by `Router.router.events.on` and I had to place it inside `componentDidMount`)
* Make sure lastAppProps always have some value.
* Revert "Make sure lastAppProps always have some value."
This reverts commit b4ae722d9c1a4460e17dbdc041b111cbd492b2aa.
* Throw an error, if we found an empty object from getInitialProps.
* Add proper tests for getInitialProps empty check.
* Move route loading logic to a common place.
* Add router events.
* Add EventEmitter's core API methods.
* Add example app for loading events and docs.
* Fix some typos.
* Get rid of Router.ready()
* Remove events package.
It's already shipping with webpack.
* Handling aborting properly.
* Expose simple attribute based events listener API.
Removed the proposed event listener API from the public API.
* Remove error logged when there's an abort error.
There are many other ways to capture this error.
So, it doesn't look nice to print this always.
* Change router events to pass only the current URL as arguments.
* Add a section about Cancelled Routes to README.
