Use CADisplayLink instead of NSTimer, dataSource updates

[zadr]

• CADisplayLink is a more high-resolution timer, that fire on every screen vsync (every 16-17ms), as opposed to NSTimer's runloop-based operation (which only fires every 50-100ms).
• Make collectionView:itemAtIndexPath:willMoveToIndexPath: optional, and, add a matching collectionView:itemAtIndexPath:didMoveToIndexPath:

[lxcid]

Sorry I took so long to reply. I like this update except for one small issue. Because with NSTimer, we can "assume" interval between callback to be 60 fps (as I hardcoded it) and since CADisplayLink is more dependent on the refresh rate of the screen, I felt it need a way to track the interval between frames.

I'll still merge this nevertheless but I'll create a new issue regarding my concern. Thanks for the PR!