[ios] collection view paging bug

http://stackoverflow.com/questions/13228600/uicollectionview-align-logic-missing-in-horizontal-paging-scrollview

http://stackoverflow.com/questions/13492037/targetcontentoffsetforproposedcontentoffsetwithscrollingvelocity-without-subcla

override

flowlayout’s method

targetContentOffsetForProposedContentOffset