Added z-index:1 to list-view + brought back transform3d positioning, removes CSS requirements

Fixes https://github.com/emberjs/list-view/issues/54

- Moved applyTransform from ListView to ListViewMixin since VirtualListView
  also uses it.
- Brought transform3d based positioning back into applyTransform
- Added z-index:1 to ListView in order to fix scroll-bar layering on webkit
- Added position:absolute to ListItemViewMixin to remove CSS requirements
- Removed ListViewHelper (no longer needed)
- All tests seem to pass

Note: This does not fix #48 (inertial scrolling conflict with mouse events
      on webkit).