Update README for callbacks

Some other minor types and added TODO item for patching
other things than only the TTL
First infrastructure for callbacks
Add Callback to Service

Some groundwork for callback, callback will be separately put in the
registry, but linked from each Service.
Extend the registry for callbacks

Add {Add,Remove}Callback to the registry so we can set these callbacks
Add AddCallbackCommand to command.go
Add callbacks to the services.
Put these CommandName functions on a single line
Add RemoveCallbackCommand
Make it compile again
Add seperate callback file
dont add it
Put the callbacks directly in the Service definition