PYCBC-225: Expose master-only observe

Change-Id: I9e189ac69fd104b9bc8bab2710aa0ee1c8acbebf
Reviewed-on: http://review.couchbase.org/33410
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Reviewed-by: Dave Rigby <daver@couchbase.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>