Allow changes_only tracking of embedded resources

[rgraff]

Embedded resources always contain a complete snapshot of the new embedded resource. First it should abide by the change_tracking_mode and when set to :changes_only should only include the change. It should also be configurable to be overridden if you want changes only for the resource but snapshots for the embedded resource.

paper_trail do
  embedded_resources_change_tracking_mode :changes_only # defaults to the value of change_tracking_mode
end