Removed chapter slide-cover "blackout" style

Due to performance issues with CSS3 transforms, the Octocat chapter cover slide styling has been simplified.
![Screen Shot 2013-03-21 at 5 13 32 PM](https://f.cloud.github.com/assets/352082/288622/1f9de64a-927d-11e2-9842-e63695e9773e.png)