New terms, definitions and model

[Tony Linde]

> My turn to catch up... I'll try to answer various emails and 
> then cut a new version of the model page and diagram.

I've added terms, definitions and new cuts of the model to the wiki page:
  http://www.ivoa.net/twiki/bin/viewauth/IVOA/RegistryModel

Some of my definitions might conflict with the RM document but take them as
suggestions for revisions to that document for now.

Cheers,
Tony.