Added backface-visibility w/ vendor prefixes

This is to fix a rendering issue with Chrome where CSS3 animations
cause a 1px shift of some elements and a colour shift whilst the
animation is in motion.
Previous fix tweaked for Compass

Changing the previous fix so it uses the Compass method and added a
comment to identify why it's there.
Merge branch 'master' of github.com:atomworks/Gumby into atomworks-master
Merge branch 'develop'