Clean up mixins as per autoprefixer recommendations

This corrects most of the incorrect cross-browser flexbox directives.

Reference: https://github.com/ai/autoprefixer
Reference: http://jsfiddle.net/LE5ym/2/show/light/
Remove @-moz-keyframes as per autoprefixer recommendations

Reference: https://github.com/ai/autoprefixer

Reference: http://jsfiddle.net/LE5ym/2/show/light/
Add .box-shadow mixin
Rework flexbox mixins to use 'when' guard
Merge pull request #15 from ptb/mixins

Clean up mixins as per autoprefixer recommendations