Add "What is a commit" AI source diagram

Including new source Illustrator file used to build the "What is a Commit" diagram in the Foundations deck.

![screen shot 2013-10-10 at 5 29 41 pm](https://f.cloud.github.com/assets/352082/1311244/e9deda10-3203-11e3-8d1b-31e54b06bd6a.png)