Adding a TrackableManager with better handing of get/update_or_create methods.

Signed-off-by: Raphael Lullis <raphael@lullis.net>
Synchornizable classes can provide a map of local-to-remote attributes. Should make things easier when merging local and remote objects.

Signed-off-by: Raphael Lullis <raphael@lullis.net>
Work on load_users. Not using get_or_create, but can detect when the remote user needs to be merged or read from remote.

Signed-off-by: Raphael Lullis <raphael@lullis.net>