Missing doc for KajsonManager, ClassRegistryAbstract, and ClassRegistry

[bpietropaoli]

Hey there,

The doc is missing for these three classes. I think they should be documented, in particular because the class registry can be used to create classes on the fly as per the README.

Kind regards,
Bastien

[lchoquel]

Fixed by PR #43 regarding KajsonManager.

Docs for ClassRegistry were actually already there, maybe not published properly.