implement frontend autoupdate

Only the unread counter is updated right know.

The AutoUpdate Feature is designed on the premise of don't wasting resources. A
distinction is made between updates when Miniflux is visible or hidden.

To determine the visibility status, the Page Visibility API is used. The API is
available starting with Chrome 33, Firefox 18 and IE10. [https://developer.mozilla.org/en-US/docs/Web/Guide/User_experience/Using_the_Page_Visibility_API]
As IE9 returns an undefined, it doesn't break the compatibility at least.

If Miniflux is hidden the updating stops after the first time an update is
found. Only the page title is updated to notify the user. If Miniflux gets
visible again (visibilitychange event fires), a fresh unread counter is queried
from the database and the DOM gets updated.

If Miniflux is visible the DOM is updated immediately and a new round of update
checking starts.