PYCBC-86: Implement delete command

Change-Id: I0dce0c3c702af0cf70a00b69244e0696d1ebd22d
Reviewed-on: http://review.couchbase.org/25906
Reviewed-by: Matt Ingenthron <matt@couchbase.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Volker Mische <volker.mische@gmail.com>