Added portal animation from ZEN BabyBook app

Kindly contributed by FreddyF. Original source here:
https://github.com/FredddyF/BabyBook-v2--iOS7-/blob/master/ZEN%20BabyBook/ZENPortalAnimationController.m
Renamed portal animation adding the 'CE' prefix
Portal animation reverses properly

A few minor changes:
 + The forward animation uses snapshots. These have to be replaced with the real views when the animation completes
 + The reverse animation snapshots a view which hasn't been added to the hierarchy. This needs to be added in order to snapshot properly (you get unexpected behaviour otherwise)
Added documentation for portal animation