Make result repr nicer

- Don't append strings
- Only print out relevant fields
- Types can (and should) declare the fields which are relevant for them
  (as some types may contain other fields, but those fields would often
be empty)

Change-Id: I99ab162547e4c8b488698eb6629229cbca552919
Reviewed-on: http://review.couchbase.org/26706
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Mordechai Nunberg <mnunberg@haskalah.org>