Enable TOC "open" by default

Suggested via https://github.com/github/training-kit/issues/263, this provides the TOC open/on by default.

![screen shot 2014-11-09 at 7 10 14 pm](https://cloud.githubusercontent.com/assets/352082/4970323/baf434cc-687e-11e4-96b0-b55d0155362e.png)