Feature: Add property ton enable panning over UIControls

I've added a property to PSStackedViewController to enable panning over UIControls. It is set to NO by default to preserve the previous behavior in order not to break existing apps.

I'm not sure about the name of the property, so feel free to suggest a better one.

I've tested it with UIButtons and UISlider. The panning takes over the slider when the view is still moving, but otherwise it works fine. Was there another UIControls that was giving problems?

Anyways I hope my contribution is worthy (although small) ;) And thanks for all the great projects you give!