[css3-flexbox] Flexbox issues

1. box-pack, box-align, and box-direction are insufficiently
direction-agnostic in their descriptions.  While they do say something
about what direction or edge they are referring to based on the
box-orient value, the actual algorithm descriptions tend to assume a
horizontal box-orient.  These need to be reworded slightly to be
properly agnostic about the direction (or conversely, to be properly
*explicit* about the direction, whichever is clearer).

2. `box-align: stretch` does not allow sufficient control over the
alignment of the box contents after the height is increased.  I guess
the children just act like normal boxes, and top-align themselves?
This prevents us from, say, baseline-aligning boxes but still wanting
a background/border to be the same height on everything.  I suggest
that either 'stretch' should be a valid value *in addition to* the
other values, or that we should define vertical-align affecting
flexbox children similarly to how it affects table cells.

~TJ