PYCBC-92: User-defined transcoders

This provides the transcoder interface, some more documentation on how
to use it, and an efficient C-based base class.
Change-Id: I2e24aa2431110519575e8ac0c224a511995123d1
Reviewed-on: http://review.couchbase.org/26391
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Mordechai Nunberg <mnunberg@haskalah.org>