Added fadeout to gh-popover
Added fadeout to gh-popover

Closes #2868
- Uses jQuery's `.fadeOut` whenever a popover is closed.

- Reordered `gh-popover`'s code into something a bit more logical and, if
  I may, pretty
- Renamed `open` property into `isOpen`. `isOpen` should only be
  manipulated via `close()` and `open()`
- Added `closing` property to help track state in the case of rapid clicks
  on a popover's button, allowing us to abort
` Added `open()` function

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