Skeletor

I believe this should be feature complete. It's essentially the SASS version of Skeleton plus Bourbon and mixins for flexbox.

@bertrandk @neilff @lustandfury @SethDavenport @claudiareinoza @abmohan @AbdellaToronto could you please review. 

I tried to use BEM syntax for all class names. Let me know if you spot any anomalies.