Adding mockups (and a hippo) to the Ruby on Rails Tutorial book

I've added a bunch of mockups (made using the excellent Mockingbird wireframing application) to Chapters 5–11 of the Ruby on Rails Tutorial book. As with real-life mockups, the book mockups help motivate the HTML interface by giving a preview of what is to come. They also serve as a gentler introduction to some of the more test-heavy sections of the book. Plus, they gave me a chance to include pictures of a hippo and a shark in the tutorial, among other things, and it's hard to argue with that. Check them out when you get the chance!