Fix "targetContentOffsetForProposedContentOffset:withScrollingVelocity:" not called

In your code, you are using _delegate for both PSTCollectionViewDelegate and UIScrollViewDelegate. I separated them into _delegate (UIScrollViewDelegate) and _collectionViewDelegate(PSTCollectionViewDelegate).

_delegate is set to PSTCollectionView itself, and it forwards targetContentOffsetForProposedContentOffset: to PSTCollectionViewLayout.

Cheers!