3.1.9 - fix z-index and scrolling enhancement

New options
- nativeparentscrolling (default:true), detect bottom of content and let parent to scroll, as native scroll does - close #176

Changed features
- Smoothest scrolling with mousewheel

Fixes
- zindex=auto, recurve check for use correct zindex value - fixes #149
- horiz scrollbar position issue - fixes #154