- fix targetContentOffsetForProposedContentOffset:withScrollingVelocity: not called
- relay UIScrollViewDelegate methods to _collectionViewDelegate
Merge pull request #170 from lono175/master

Fix "targetContentOffsetForProposedContentOffset:withScrollingVelocity:" not called
Preserve ivar layout.
Fixed createPreparedCellForItemAtIndexPath:layoutAttributes: not applying attributes
Fixed nil return from layoutAttributesForElementsInRect resulting in all cells being removed.
Merge pull request #179 from tyrone-sudeium/bugfix/nil-layout-attributes

Behaviour when layoutAttributesForElementsInRect is nil
Merge pull request #178 from tyrone-sudeium/bugfix/attributes-not-applied

CreatePreparedCellForItem... doesn't apply attributes.