Fixed an issue with modal controller on iOS6+

Hey folks,

This pull request fixes an issue when the SK controller is attempted to be presented on the root view controller, while the root view controller is modally presenting another controller.
The current implementation fails to present the SK since the root view controller already has a modally presented controller.

The fix is fairly simple: iterate through "presentedViewController" when fetching the root view controller, and the same when dismissing it.