Demonstrate reuse of subclasses and refactoring an abstract base clas…

…s to use

composition.

Example of injecting dependencies, step-by-step

* Recreates an example where dependencies were not injected
* Refactors to use dependency injection

Small commits representing individual steps of Extract Class refactoring

This tag preserves a series of commits that were split up from commit fd6cd8d
so that they can be referenced from the book.