Updated issue tracking doc to include objections and responses

I've updated the issue tracking doc to include objections and responses.

There is a summary of the objections at [1].  Each objection links to 
the relevant issue (except completeness of entailment rules which may be 
  about to go away) and its late.

The relevant issue contains the WG response to the objection.

I've taken into account feedback from DanC, which includes stating the 
conflicting requirements on the datatypes objection and some (much 
shorter) text on the I18N issue.

Please review.

The intent is that the request to advance will refer to the objections 
in the issue doc.

Brian