Added more test scenarios for HistogramExample
- HistogramExample: fixed an error in the runExample method: initialized result to false, and switched to workflow factory (instead of workflow engine factory).
- HistogramExampleTest: added more test scenarios.
- DummyWorkflow: added options to configure the value that will be returned by the run method and whether an exception should be thrown by the run method.
- BaseExample: added a constructor with a workflow factory as its single parameter.