[css3-flexbox] Flex-align possibilities

I'm merging the changes that Alex and I agreed on into the Flexbox
spec right now, and I'm still not quite sure what I want to do about
flex-align.  I've got a couple of choices, which I've illustrated
here: <http://www.xanthir.com/diagrams/flex-align-diagrams.html>.
(Use Firefox - Webkit has a bug regarding box-sizing and percentage
heights on elements in table-cells that screws up the 'stretch'
diagrams, and I dunno about other browser's box-sizing support because
I don't have them on my current box.)

As you can see by the diagram, there a bunch of possibilities.  At
minimum, we should support 1, 6, 13, 14-17, and either 2, 3, or 7.
The other cases may or may not be useful; align+switch cuts out all
the other ones.

So, bikeshed time.  What seems best?  They're all roughly equal from a
spec-writing perspective.

~TJ