Added fadeout to gh-popover

Closes #2868
- Uses jQuery's `.fadeOut` whenever a popover is closed.

Death to the ancient issues!
That'd work much nicer, save for one problem.  When !open, we can't fade out because it's not open anymore ;)

For that to work (and I agree it's a prettier & safer solution than just a stinkin' comment), we'd have to indirect the open class-binding to something else.