PYCBC-135: Allow transcoder to be a class

Change-Id: I161e6d739a3bcd4daaf472fe4d6f680b8546f594
Reviewed-on: http://review.couchbase.org/26985
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Mordechai Nunberg <mnunberg@haskalah.org>