Check whether the panning is horizontal before opening side menus

Hi Mike,

I have added a check in ```gestureRecognizerShouldBegin:``` whether the panning gesture is horizontal panning before handling it. I think it is useful in the case the main view controller has a scroll view, e.g UITableViewController, the panning up and down on the table won't open the side menus.

Again, thank you for your project =)

Cheers,
Thuy