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)

AnimationControllers/CEPortalAnimationController.m

@@ -69,8 +69,11 @@ - (void)executeForwardsAnimation:(id<UIViewControllerContextTransitioning>)trans
 
                          // remove all the temporary views
                          if ([transitionContext transitionWasCancelled]) {
+                             [containerView addSubview:fromView];
                              [self removeOtherViews:fromView];
                          } else {
+                             // add the real to- view and remove the snapshots
+                             [containerView addSubview:toView];
                              [self removeOtherViews:toView];
                          }
 
@@ -89,6 +92,10 @@ - (void)executeReverseAnimation:(id<UIViewControllerContextTransitioning>)transi
     // Add the from-view to the container
     [containerView addSubview:fromView];
 
+    // add the to- view and send offscreen (we need to do this in order to allow snapshotting)
+    toView.frame = CGRectOffset(toView.frame, toView.frame.size.width, 0);
+    [containerView addSubview:toView];
+
 
     // Create two-part snapshots of the to- view
 
@@ -132,6 +139,7 @@ - (void)executeReverseAnimation:(id<UIViewControllerContextTransitioning>)transi
                              [self removeOtherViews:fromView];
                          } else {
                              [self removeOtherViews:toView];
+                             toView.frame = containerView.bounds;
                          }
 
                          // inform the context of completion