New flex box

[zernie]

Can be fxd new-flex-box branch merged to a new-flex-box/master?

[plapier]

@ximiks See the comment I left on issue #115

If the 2012 spec of flex-box is implemented into at least one other browser with vendor prefixes, like firefox, then I think it makes sense to merge it in.

Otherwise, the mixins would be pointless to introduce if browsers implement flex-box straight to spec with no prefixes.

[zernie]

Last Flexbox spec (2012) are supported by:

• Chrome 21+ (-webkit-)
• Opera Mobile 12.1+, Opera 12.10+ (without prefixes)
• Firefox 18 + (without prefixes)

Unfortunately, IE 10 supports 2011 spec version.

Anyway, your code from new-flex-box branch doesn't work. There some mistakes with prefixer mixin using:)