Changed implementation to use affine transform translations instead of frame offsetting

For whatever reason sometimes in iOS 8 frame offsetting wasn't working so you'd end up in a state where the nav bar was visible but the nav bar items were invisible (alpha = 0). In the debugger, even though the nav bar offset was being calculated correctly, -setFrame: in -setBarOffset:animated: wasn't actually changing the frame on the screen, or something else was resetting it. Switching to affine transforms seems to have fixed this.