Flexbox

Add `display: flex` polyfills to `display.styl` and all the other mixins: `flex-*`, `order`...

References:

- http://caniuse.com/flexbox
- http://css-tricks.com/old-flexbox-and-new-flexbox/
- http://coding.smashingmagazine.com/2013/05/22/centering-elements-with-flexbox/
- https://github.com/visionmedia/nib/blob/master/lib/nib/flex.styl
- https://github.com/chriseppstein/compass/blob/stable/frameworks/compass/stylesheets/compass/css3/_box.scss
- http://dev.opera.com/articles/view/advanced-cross-browser-flexbox/
- https://gist.github.com/cimmanon/727c9d558b374d27c5b6
- https://gist.github.com/cimmanon/4461470
- http://msdn.microsoft.com/es-es/library/ie/hh673531(v=vs.85).aspx
- http://msdn.microsoft.com/en-us/library/ie/dn265027(v=vs.85).aspx
- https://github.com/mastastealth/sass-flex-mixin/blob/master/flex.scss
- http://codepen.io/adamjohnson/pen/savzI