interface IPublishedDataStore.
Simplified the interface structure and removed the GetDocumentProperty method as it is not needed and i think existed
before we updated the codebase to not rely on xml.
moving forward will open up many nice opportunities. This will be used for the media accessor in the new DynamicDocument.
Renamed XmlPublishedContentStore to just DefaultPublishedContentStore.
Made GetProperty an extension method for IDocument and removed from the interface as it is not needed there.