Check proper x velocity when pan ends

Without this change, dragging the top view right while right-anchored (`shouldAllowPanningPastAnchor == YES`) will activate a reset and the top view will slide left, back to its original position.

![ios simulator screen shot 2013-07-16 1 51 03 pm](https://f.cloud.github.com/assets/153104/806806/1e637e68-ee42-11e2-8d19-7c759595ea26.png)