Allow bench.py to use the transcoder

Change-Id: Id4f7a9eb1d6849f3c175867fefab92faa2941a96
Reviewed-on: http://review.couchbase.org/26390
Reviewed-by: Mordechai Nunberg <mnunberg@haskalah.org>
Tested-by: Mordechai Nunberg <mnunberg@haskalah.org>