Delegate instantiation of SCXML document workflow model objects to factory methods to make customizations easier

Right now the DocumentWorkflowImpl and DocumentHandle classes use direct instantiation of the DocumentHandle, DocumentVariant and Request model objects.
This makes it difficult to customize the Document Workflow without forking all of the code.

To make this easier, we'll:

• provide an optional factory class to create a DocumentHandle instance
• provide factory methods in the DocumentHandle itself to create DocumentVariant and Request instances