Add grpc transport (#67)

diff --git a/system_tests/test_grpc.py b/system_tests/test_grpc.py
new file mode 100644
index 0000000..0db9f1d
--- /dev/null
+++ b/system_tests/test_grpc.py
@@ -0,0 +1,40 @@
+# Copyright 2016 Google Inc.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import google.auth
+import google.auth.credentials
+import google.auth.transport.grpc
+from google.cloud.gapic.pubsub.v1 import publisher_api
+
+
+def test_grpc_request(http_request):
+    credentials, project_id = google.auth.default()
+    credentials = google.auth.credentials.with_scopes_if_required(
+        credentials, ['https://www.googleapis.com/auth/pubsub'])
+
+    target = '{}:{}'.format(
+        publisher_api.PublisherApi.SERVICE_ADDRESS,
+        publisher_api.PublisherApi.DEFAULT_SERVICE_PORT)
+
+    channel = google.auth.transport.grpc.secure_authorized_channel(
+        credentials, target, http_request)
+
+    # Create a pub/sub client.
+    client = publisher_api.PublisherApi(channel=channel)
+
+    # list the topics and drain the iterator to test that an authorized API
+    # call works.
+    list_topics_iter = client.list_topics(
+        project='projects/{}'.format(project_id))
+    list(list_topics_iter)