Add delegate methods for controlling visual changes and movement of the ...

...current dragging item.

This adds these delegate methods (all optional):

    // Called as dragging starts from animation block to allow custom visual adjustments to the view.
    - (void)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout *)collectionViewLayout adjustCurrentViewForDragAnimated:(UIView*)currentView;

    // Called when the draggable view moves, giving the delegate a chance to control how it has moved.  Useful for restricting movement to a single direction or constraining it to a specific boundary.
    - (CGPoint)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout *)collectionViewLayout adjustTranslation:(CGPoint)translation forDragOfCurrentView:(UIView*)currentView;

    // Called as dragging ends from animation block to allow reversing the animations applied when dragging started.
    - (void)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout *)collectionViewLayout adjustCurrentViewForDropAnimated:(UIView*)currentView;

Not implementing any of these methods provides the default behavior as it is prior to this change.