Week of Aug19 changes

[duglin]

• fix "attributes" in "messages" model for CE format
• s/schemauri/schemaurl/ for now, to align with the spec
• remove 'readonly' attribute from model Resources
• add 'readonly' attribute to Resource instances - server controlled
• s/time/timestamp/ where appropriate
• update codegen:
  • s/createdon/createdat/
  • s/modifiedon/modifiedat/
  • add "origin" as a common attribute
  • remove "format" as a common attribute

Fixes #148

[duglin]

Approved on the 8/22 call