Tweaks to appearence transitions handling on setCenterViewController* methods

Relates to issue 138: https://github.com/mutualmobile/MMDrawerController/issues/138

- Fixes missing `[oldCenterViewController willMoveToParentViewController:nil]`
- Fixes missing edge case on setCenterViewController:with(Full)CloseAnimation which resulted in view*Disappear methods not being called
- Fixes edge case which resulted in view*Appear methods being called twice
- Reorganises begin/endAppearenceTransition methods to have a more systematic calling order (begin's on both VCs called before end's)