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`)
