add 'couchbase.tests.cases' to setup.py

Change-Id: I0a31a3e2b2bdab126927fe69d57031d5518ac0cb
Reviewed-on: http://review.couchbase.org/29818
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>