Added fadeout to gh-popover

Closes #2868
- Uses jQuery's `.fadeOut` whenever a popover is closed.

Death to the ancient issues!
> click the open button while animating out

Very good point. Not sure how to handle cases like that. Best keep it as a jQuery animation for now then, i think. It working is more important right now.

At some point, popovers will move to Ghost-UI where we can have a fresh look at it all. :)