Added fadeout to gh-popover

Closes #2868
- Uses jQuery's `.fadeOut` whenever a popover is closed.

Death to the ancient issues!
Just to quickly chime in, if there's anything that needs chancing in Ghost-UI, just shout! I'm thinking if some wizardry with `animationend` would be a good idea?

Click close -> Add class to fade out -> when animation ends, hide the popover -> done.