PYCBC-115: Add touch command

Change-Id: Ia41c3b4e38f9c90a9ad33bef4ec3f6cfdc437888
Reviewed-on: http://review.couchbase.org/26337
Reviewed-by: Pavel Paulau <pavel.paulau@gmail.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Mordechai Nunberg <mnunberg@haskalah.org>