switch gist code to use full user, added new fields

Using https://api.github.com/gists/6305249 as a reference, I've updated http://developer.github.com/v3/gists/#get-a-single-gist to 

 - show the new fields
 - display the `FULL_USER` field instead of `USER` wherever relevant

Let me know if there's anything else I've missed for this change.