Make BillGenerator method static

 - There is no need for the method to be an instance method
Make classes with only static methods have private constructors

 - So that they cannot be instantiated
 - nor subclassed
Refactor BillGenerator to be Bill object
 - So that tests can actually get the values of the calculated bill
Refactor BillingSystem to return Bill objects instead of printing
 - So that tests can check values
Refactor sub-classes of CallEvent to take timestamps
 - And for BillingSystem to insert the time now as necessary
Remove extraneous comment
Merge pull request #1 from rmbennett/test-refactor

Refactor of Code for Test Purposes I