Adds a unit test generator to pay_dirt's thor task

These changes to the pay_dirt.thor file will

- have the generator build a unit test in `test/unit/.../*_test.rb`
- the unit test will require "minitest_helper.rb" unless --test_framework:anything_else is given
- the generator will append a line to "minitest_helper.rb", requiring the newly generated service object (--test_framework:anything_else will result in appending the line to "test_helper.rb")

By default the service object will have 100% test coverage...