Update references for data integrity proofs

[clehner]

Fix #44

• 32dc033 Change linked data proof to data integrity proof
  • Remove definitions of linked data document and proofs. Reference data-integrity-spec instead.
  • Remove mentions of linked data, except when the document is known to be JSON-LD, i.e. for the canonicalization hash.
• e769be2 Update references
  • Rename "Linked Data Proofs" to "Data Integrity".
  • Update Security Vocabulary publisher to CCG.
  • Use "Draft Community Group Report" status consistently. "CGDRAFT" is not recognized as a status by specref.