[css3-flexbox] flex-align: stretch and max

The spec doesn't define what should happen when a flex-align:stretch flex
item hits a max-width or max-height.  For example, in a horizontal flexbox,
there might be a max-height on a flex item that prevents it from stretching
to fill.  I think we should obey the max-height, but align the item to the
start.  The other option seems to be to center align, but start seems a bit
simpler and more consistent with the non-auto case.

tony