Use CSS translate instead of absolute positioning on wrapper and sidebar for better performance

From Paul Irish's [blog post](http://www.paulirish.com/2012/why-moving-elements-with-translate-is-better-than-posabs-topleft/) about using CSS translate on moving elements, I updated positioning of `.wrap` and `.sidebar` from using `left` (and `right`) property to `translateX` and did some quick test to see if it helps.

toggled sidebar for a couple times.

before: using `left` and `right` positioning
![css-left-right](https://f.cloud.github.com/assets/911894/2030700/c12502be-88f9-11e3-8958-35d66826673f.png)

after: using `translateX`
![css-transform-translate](https://f.cloud.github.com/assets/911894/2030706/e3d69598-88f9-11e3-8ccd-27c80400fde0.png)

as you can see, using `translate` can reduce painting time of toggling sidebar. It also looks "smoother", noticeable on mobile (tested on iPhone 4s, mobile Safari)