Pull Request for Exercises 0 and 1

This pull request covers all slides up until #18. So far, it took me about 1 hour to go through the slides in my head and complete the exercises. 

I've changed instructions where I thought the task could be more clear, or where a hint might be needed. I also made some additions to the unit testing. It would be interesting to see how we could better incorporate the unit testing into the entire track, like maybe having the students update their unit tests per exercise if it's not too much work? Or throwing it in as a bonus - make these tests pass. 

Please note: I did not create zips of the new exercises, as I was waiting for your feedback on my changes.

Update: replaced bookshelf_screenshot.png