Animate backCardView should happen during swipe

[acegreen]

I think it looks more appropriate when we perform the following as the swipe is taking place. Also it needs to factor in the motion (as tinder does when you begin swiping)

backCardView = self.popPersonViewWithFrame(self.backCardViewFrame())

// Fade the back card into view.
if backCardView != nil {

        self.backCardView.alpha = 0.0
        self.view.insertSubview(self.backCardView, belowSubview: self.middleCardView)
        UIView.animateWithDuration(0.5, delay: 0.0, options: .CurveEaseInOut, animations: {
            self.backCardView.alpha = 1.0
            },completion:nil)

    }