Proposed an animation fix

[congliu0704]

Summary

1. Fixed scroll-to-bottom condition so that both sides are relative y-coordinates. cell.frame.maxY > tableView.frame.maxY should be cell.frame.maxY > tableView.frame.maxY + tableView.contentOffset.y
2. Enabled scrolling to the top of the cell, when some of top part of this cell is across the upper edge of the table view (that is, hidden) and is tapped to unfold.