updateDocument() method for model-instance-client

[mzkrasner]

• Adds a method to post an update to a document that optionally obtains docstate first
• Uses existing getDocumentState() method if StreamState is not provided in arguments
• Relies on existing postData() method which uses the createDataEvent() method to access JSON diffing
• Requires newContent and currentContent to be defined - throws an error if currentContent is either not provided or cannot be fetched from StreamState