Fix bug where animated argument was ignored

anchorTopViewTo*Animated: and resetTopViewAnimated: is ignoring the animated
argument. In practice is not the ECSlidingViewController but rather the
ECSlidingAnimationController (default implementation).

Apple also recommends to check the isAnimated property of the
UIViewControllerContextTransitioning delegate.

Fix bug #327

Signed-off-by: Felipe F. Tonello <eu@felipetonello.com>