onFinish impossible to reach in stack

[jklingen92]

Because the PanHandlers are removed when you're on the last card in a stack, it's impossible to advance to the onFinish code. I've opened a pull request that adds a swipeThroughStack prop to the component which allows you to swipe on the last card in the stack. If your onFinish function does nothing, the last card will just reappear. (#38 )