Fixes #119
Fixes #325
Ref #241

Fixes #311.

Change-Id: I334b9e22e748d96a62b0c7f59690c1a998a3c802

This tool help to identify fields being returned by the GitHub API that
are not currently being mapped onto our Go data types.  Some of these
may be deliberate omissions, so these checks should only be treated as
an aid in identifying these fields, not as an absolute list of fields to
be added.