scroll position management for page navigation

Addresses https://github.com/yahoo/flux-router-component/issues/32

@redonkulus @mridgway @ptomasroos
I think this could be a little simpler since we only care about when scrolling has finished.

```js
var timeout;
window.addEventListener.on('scroll', function (e) {
    clearTimeout(timeout);
    timeout = setTimeout(function () {
        // do state stuff
    }, 100);
});
```