Clarify exactly which properties the LRS MUST (or SHOULD) omit when responding to a GET Statements request with format=ids

[garemoko]

See https://github.com/adlnet/grail-issue-tracker/issues/61#issuecomment-272418796

I'm suggesting that the following is what's required of the LRS, mainly because this is what Watershed does today and it seems reasonable to me.

• For agents the name property is omitted
• For verbs the display property is omitted
• For activities the definition property is omitted.

The point of contention is whether or not the LRS MUST omit the objectType property when the actor contains an agent (not a group). I'd argue that the LRS SHOULD NOT but MAY.

[andyjohnson]

Per the 1/18/17 call, come up with a list of what is not allowed and replace the vague text in Communication.md.