Data model spec okay for review

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

The data model specification has undergone major editorial surgery, mainly
having to do with aligning all the new terminology.

The document now more clearly distinguishes among an identity profile (the
general term), the identity profile model (the generalized model the
document defines), and an identity profile expression (a specific instance
of the data model as represented in JSON, JSON-LD, or WebIDL).  Ditto for
entity credentials.

Known items remaining that I will work on tomorrow and/or Monday:
- fix the JSON-LD examples and signatures to be closer to realistic
- put in the proper reference for the use cases document
- fix the terminology section where, for some reason I don't yet
understand, the terms 'identity' and 'credential' are being included even
though I believe I have erased them from the document as stand-alone terms.

All feedback is welcome, but I would particularly like to know if I missed
correcting any terms used in the text to our newly agreed upon versions.

-- dan