Horizontal transitions

The more I played with Stack Nav, and especially on larger devices, I realized how difficult it could be to reach to the top nav bar to perform a dismissal gesture. The alternative, tapping the top left icon to dismiss, is even harder.

I've made all view controllers animate in from the right side of the screen, and pop out by sliding back to where they came from. This affords a more intuitive visual/spatial hierarchy, more similar to that found in the stock `UINavigationController`. Because of that, we can also take advantage of the now ubiquitous swipe-from-left-edge to go back a level in the stack. Yay!

TODO: Planning on adding an optional PCStackViewController method to let the controller know whether to use a vertical or horizontal animation. Vertical animations are often found for content creation view controllers and other view controllers that shouldn't allow for gestural dismissal. This makes most sense for, like I said, content creation, where you want to request some sort of confirmation from the user before potentially deleting something they may have typed or created. View controllers that navigate vertically will only allow for dismissal via an explicit call to stackController.