z-index overhaul. relative above static; negative z-index

1. when stacking without z-index, positioned > floated > normal flow
2. supports negative z-index
3. new stacking context formed when opacity < 1 (standard)
4. new stacking context formed for position:fixed (no standard yet, done in mobile webkit and chrome 22+)
coding optimizations
passes z-index8 non-positioned element with opactiy < 1

The MDN article Understanding_z_index/Stacking_and_float is wrong about this
Refactoring