Update logo to 'Teaching'

This addresses @matthewmccullough's [request to update the logo](https://github.com/github/teach.github.com/issues/133). 

![screen shot 2013-04-29 at 7 29 36 pm](https://f.cloud.github.com/assets/416727/442209/da2cd928-b13e-11e2-9818-89d943b200ad.png)