Add $enable-null-transform-hack to disable null transform hack

Doesn't set all the `transform: translate3d(0, 0, 0);` variations if set to false, defaults to true

It was creating a bug where I had a grey line sometimes around my slides

![screen shot 2014-09-26 at 4 17 22 pm](https://cloud.githubusercontent.com/assets/129149/4426992/24e195b0-45ba-11e4-8bd8-70e010af9134.png)