Refactor exercises based on Jesse's feedback

This provides executable javascript files that a student may run and work through incrementally to explore the problem space.

Notable changes are:

1. Instructions are duplicated throughout the files sometimes
2. Using the words "use case" to describe the console.log statements.
3. Splitting it the README into 4 different sets of exercises.

@jfarmer @jcdavison Thoughts? I'd like to get moving forward on refactoring the rest of the materials as well.