Add subclass to demonstrate inheritance and overriding

[jody]

As an instructor, I want a subclass of HelloWorld that can be used to demonstrate inheritance and overriding.

Acceptance Criteria:
√ A subclass of HelloWorld exists
√ The subclass demonstrates calling an inherited method [partial; could use enhancement]
√ The subclass demonstrates overriding an inherited method
√ There is a unit test class for the subclass
x Tests within the unit test class exercise the functionality of the subclass [see: Issue #36]

[jody]

Needs actual unit test methods.
Would benefit from additional demonstrations of inheritance.