Add (final) Flexbox implementation (2013)

References:
- http://www.w3.org/TR/css3-flexbox/#property

I did not update the allowed values for min-width and min-height,
because this change was dropped from the Flexbox spec:
- http://dev.w3.org/csswg/css-flexbox/#changes
- http://lists.w3.org/Archives/Public/www-style/2013Mar/0137.html
Update properties for Flexbox 2009

Removed unprefixed box and inline-box from display

Replaced unprefixed box-* properties with moz and webkit prefixes.
Add Flexbox 2012 implementation (IE10)

References:
- http://msdn.microsoft.com/en-us/library/ie/hh673531(v=vs.85).aspx
- http://www.w3.org/TR/2012/WD-css3-flexbox-20120322/

"-ms-flex" is defined (see reference) as
   [ <positive-flex> <negative-flex> <preferred-size> ] | none
   ^ <number>      ^ <number>      ^ "<width> without inherit"
"<width> without inherit" expands to "<length> || <percentage> || auto",
so the final definition of "-ms-flex" is:
[[ <number> <number>? ] || [ <length> || <percentage> || auto ] ] | none

The other values are taken verbatim from the references.
Add webkit-prefixed properties of Flexbox 2013
Merge pull request #91 from Rob--W/flexbox-2013

Add standard Flexbox properties (2013)