Split longer notebooks into more reasonable chunks

Some of the notebooks, as currently laid out, are pretty long. This makes the project feel more like a reference than an active learning tool. Breaking into smaller sections would probably make it easier for students and teachers to navigate, and to feel like they are making progress.

What's an ideal chunk size?
- Keep the outer hierarchy roughly the same as it is currently laid out in the [syllabus](http://nbviewer.ipython.org/urls/raw.github.com/ehmatthes/intro_programming/master/notebooks/syllabus.ipynb).
- Each major subject within the current notebooks should probably become its own notebook.
- All sections should end with Exercises, and challenges wherever appropriate.