Updated data model specification document

I have updated the data model specification draft.  As before, the document
can be found at

http://opencreds.org/specs/source/claims-data-model/


The major changes since last week are:

- added introduction

- added UML diagram for the data model
Known items still to do:
- Fix/update links and references
- Update JSON-LD examples to be either real or closer to real
- Change "TBD Credentials" to real name once we have it
- Clean/unify wording now that we have "TBD Credentials" as a term we can
use


I encourage review, especially of the new introduction and UML diagram.

-- dan