Modified scrolling to use CADisplayLink instead of NSTimer.

Much less CPU intensive and avoids having to choose a timer interval, which due
to a bug was being set a 0.001 milliseconds (10,000 times per second!)
Multplied scrollRate by a factor of 10 for faster scrolling.

See the [this discussion](https://github.com/bvogelzang/BVReorderTableView/commit/bd6f75811913ed38237b2b1ec5c8b6844552eb1d#commitcomment-4580889) for more info.

Thanks to @leocassarani for the heads up.