Link Preview Popover

This pull request adds a popover which displays a preview for the currently selected list item:

![Hacky](https://f.cloud.github.com/assets/2924878/135719/1da5c614-7130-11e2-95b0-32b0da8d1a20.png)

Since the popover is quite small, Readability is used to format the pages which are linked to.

In the popover there is also an "Open in..." button which is dynamically updated to include the name of the user's default web browser ;)