Allow left and right views to have a custom frame, giving more power to ECSlidingViewControllerLayout

Small recap of modifications:

1. I modified `underLeftViewCalculatedFrame` and `underRightViewCalculatedFrame` methods to `underLeftViewCalculatedFrameForPosition:` and `underRightViewCalculatedFrameForPosition:` to allow left and right views to control their frames when implementing `ECSlidingViewControllerLayout`.
2. I fixed a bug in `viewControllerForKey` that war returning the wrong `UITransitionContextFromViewControllerKey` when the operation was `ECSlidingViewControllerOperationResetFromLeft` or `ECSlidingViewControllerOperationResetFromRight`.
3. I updated `initialFrameForViewController:` and `finalFrameForViewController:` to call the modified calculation methods.
4. I modified the default `ECSlidingAnimationController`, simplifying it thanks to the bug fixed in 2, and so not to make any assumptions concerning the frame of left and right views.
5. This allowed me to implement a `MEParallaxAnimationController` in the TransitionFun example that implements a parallax effect when showing left and right views, simply by implementing `ECSlidingViewControllerLayout`.