PYCBC-88: Implement incr/decr

Change-Id: Ia2bccb66e4d7f5f03b67bdd327ba826dad8e07eb
Reviewed-on: http://review.couchbase.org/25918
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Volker Mische <volker.mische@gmail.com>