Draw the popover arrow using CSS

The popover's arrow is currently rendered with a pair of PNG images. This removes those images and draws the arrow using pure CSS. It looks about the same:
![arrow](http://f.cl.ly/items/1r1w0u3a3t1l2t3t1c08/Screen%20Shot%202013-08-27%20at%2010.51.43%20AM.png "arrow")

@larz0 taught me how.