PYCBC-87, PYCBC-89: implement add/replace/append/prepend

Change-Id: I54e3e49e960ac43a302e1af4c1f2bdaac9f23c44
Reviewed-on: http://review.couchbase.org/25917
Reviewed-by: Matt Ingenthron <matt@couchbase.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Volker Mische <volker.mische@gmail.com>