Started using Guice for dependency injection
- renamed class WorkflowEngineFactory to DefaultWorkflowEngineFactory.
- added the WorkflowEngineFactory interface.
- added the Guice 3.0 dependency to the Maven pom.xml file.
- added the DefaultGuiceModule class to configure Guice binding.
- added the DummyWorkflowEngineFactory, DummyWorkflowEngine, and DummyWorkflow classes for testing.
- used Guice in the HistogramExample class.
- renamed class DefaultWorkflow to BaseWorkflow.