IcingVersionable check_identical & oldData

The IcingVersionable behavior now supports the config/setting `check_identical`
  if false (default) we don't check the last saved version.

The IcingVersionable behavior now exposes the data it looks up as `$Model->oldData`;
  since it looked it up anyway, we might as well grant access to the Model.

The afterSave and saveVersion now always return a boolean (as expected)

Updated the readme and defaults to be more accurate.

Added a pro tip section to the readme