Includes additions to User, Organization, Repository, and Gist.  Some
minor changes to existing fields based on changes in the v3 API (for
example, Gist.User became Gist.Owner).  Most everything else is new
additions.

refs #103

it's not the cleanest, but it works.

instead of inspecting the JSON tags on the struct fields directly,
actually run it through JSON marshalling and unmarshalling, then compare
those results.

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.