Add stub architecture

    * Add ?stub=1 to the request to get the stub
    * BaseModel has a dumb implementation of a stub() method
      that inspects the schema and creates a sumb stub return
    * Model subclasses should probably implement their own stub()
      method to provide more meaningful stub content
    * Probably want to have different stubs for singular GET,
      plural GET, and possibly even for PUT and POST.