Repaint fixes

Hey hey,
Applied translateZ / translate3d to fixed elements to prevent browser repaint when scrolling.

These changes were originally part of a Bootstrap reset I made called "Lil' B"
http://itsjonq.github.io/lil-b/

Also added a new mixin under vendor-prefixes.less - .translateZ(@z). This targets translateZ directly.
I initially wanted to use .translate3d to target the "Z" axis for the repaint fixes.. but I believe translateZ has IE9 whereas translate3d doesn't:

http://caniuse.com/transforms3d

Hope this helps!

For you guys:
https://pbs.twimg.com/media/Bl3XWPkIUAAm66k.jpg:large