Add some basic workspace and version functions, example script to start workflow

[kshepherd]

• New Version model,
• New method to get workspace item for item (useful when creating a new version as the new version goes to workspace but the REST API returns embedded item uuid, not workspace item ref)
• Slightly extend the (still very stubby) start_workflow method so that it returns the parsed json response
• example script to show how this call all be used

This is very basic stuff, written to "make versioning work" without trying to implement the full spec.

WorkspaceItem modelling in particular can be a lot because you have all the SectionData embeds, configuration, and so on. Workflow and Claimed/Pooled Tasks are also TODO and could be more work for modelling