Popover doesn't open after some 10-30 clicks.

[Aravindhanarvi]

If the user presses the Toggle Popover Button (As in Sample Project) fastly. It stops opening at some point, may be around 10-30 clicks.

[ehardebeck]

The _popoverWindow gets into a state where isVisible is YES, but alphaValue is 0
Because popoverIsVisible is true in togglePopover, it will always attempt to close it, never opening it and setting alpha back to 1.0

This seems to be happening because of a race condition where the animation delegate is set to nil before the animation started in INPopoverWindow.dismissAnimated is finished. So INPopoverController animationDidStop is never called.

Pull request for fix: #33