Add button to toggle 50% alpha on demo views.
Rewrite pop code to provide iOS7 style animations.
Rewrite push code to provide iOS7 style animations.
Rewrite popToViewController to provide iOS7 style animations.

This refactor updates the code to rely on popViewControllerAnimated: instead of duplicating animation work.
Rewrite replaceRightViewController code to provide either iOS7 push style or fade animations.
Add iOS7 style swipe-to-navigate-back functionality.
Document iOS7 style animations and swipe-to-navigate-back functionality.
Prefer constants to defines.