PYCBC-124: Allow modification of default JSON/Pickle converters

Change-Id: I2082782ea9793cedadcfba1841d94769f633f6a8
Reviewed-on: http://review.couchbase.org/26719
Tested-by: Mordechai Nunberg <mnunberg@haskalah.org>
Reviewed-by: Volker Mische <volker.mische@gmail.com>