Refactoring MMCoverFlowLayout

- changed contentWidth and contentHeight into visibileSize and contentSize property
Added -description method to MMCoverFlowLayoutAttributes
Refactored MMCoverFlowLayout and MMCoverFlowLayer

- selected item will always be centered in the visible area of the layout (using visibleSize of MMCoverFlowLayout)
- MMCoverFlowLayer is no CAScrollLayer subclass anymore
- changed MMCoverFlowLayers sublayer hierarchy to have one CATransformLayer sublayer, the replicatorLayer will be the its only sublayer
- reflections working