Add methods to Register to support updating mappings.
Add a new command, daybook-map.

This command lets a user list the services for a mapping or add/remove services to a mapping.  It's just a super thin wrapper so you can more easily script the updating of the mappings.
Tweaked the register to support more operations.

Adding services to a non-existent key will create that key for us.  Also, you can't delete from a mapping that doesn't actually exist yet.