[css3-flexbox] auto margins

Currently the spec says to treat auto margins as a flex of 1. This seems to
me that it adds complexity without any benefit. Using margin: auto to center
fixed size blocks is a hack that we don't need to perpetuate to other layout
mechanisms that have a proper way to center elements. It simplifies both the
implementation and the use of flexbox if we treat auto margins as 0.

Ojan