Add flexbox mixins to webmaker

[k88hudson]

A custom component with utility mixins for flexbox would be very useful, let's add this --

There are some good flexbox mixin modules out there, let's just pick one and include it as flexbox.less

[k88hudson]

https://gist.github.com/jayj/4012969 would work, of if there's something in bower that's fine too

[k88hudson]

Looks good just fix up that documentation nit

[mlveis]

i fixed up the documentation

[k88hudson]

Great!