Initial and final layout attributes for CircleLayout

Apple changed UICollectionViewLayout API: they renamed `initialLayoutAttributesForInsertedItemAtIndexPath` to `initialLayoutAttributesForAppearingItemAtIndexPath` and `finalLayoutAttributesForDeletedItemAtIndexPath` to `finalLayoutAttributesForDisappearingItemAtIndexPath`. Moreover, these methods now called for *every* item in collection view, so we have to handle inserting and deleting by ourselves. I fixed CircleLayout sample a little bit.