GitHub logo in footer should link to GitHub.com

Noticed that there's no easy way to get to github.com from training.github.com. I think that GitHub's logo in the footer is a good candidate for serving that purpose. Currently the footer logo just links back to training.github.com, but I think it makes more sense to make it link to github.com. The GitHub Training logo in the header still points to training.github.com. 

![screen shot 2013-08-06 at 9 08 24 am](https://f.cloud.github.com/assets/38924/921695/d09a155c-ff03-11e2-89cc-b8c6f49439cd.png)