Add 'panGesturesEnabled' property

Thanks for a great component!

I'm using `MCPanelViewController` in an iPhone app and the pan-to-dismiss behavior is not desirable for me because

- My presented panel view controllers are always full-screen, and 
- I'm using a navigation controller with a 'Done' button that takes a custom action in addition to dismissing the panel.

I've added a `panGesturesEnabled` property that will enable (or disable) the pan gesture recognizer on the presented panel.