Add system tests for service account credentials (#51)

diff --git a/system_tests/conftest.py b/system_tests/conftest.py
new file mode 100644
index 0000000..066f805
--- /dev/null
+++ b/system_tests/conftest.py
@@ -0,0 +1,74 @@
+# 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 json
+import os
+
+from google.auth import _helpers
+import google.auth.transport.urllib3
+import pytest
+import urllib3
+
+
+HERE = os.path.dirname(__file__)
+DATA_DIR = os.path.join(HERE, 'data')
+HTTP = urllib3.PoolManager()
+TOKEN_INFO_URL = 'https://www.googleapis.com/oauth2/v3/tokeninfo'
+
+
+@pytest.fixture
+def service_account_file():
+    """The full path to a valid service account key file."""
+    yield os.path.join(DATA_DIR, 'service_account.json')
+
+
+@pytest.fixture
+def request():
+    """A transport.request object."""
+    yield google.auth.transport.urllib3.Request(HTTP)
+
+
+@pytest.fixture
+def token_info(request):
+    """Returns a function that obtains OAuth2 token info."""
+    def _token_info(access_token=None, id_token=None):
+        query_params = {}
+
+        if access_token is not None:
+            query_params['access_token'] = access_token
+        elif id_token is not None:
+            query_params['id_token'] = id_token
+        else:
+            raise ValueError('No token specified.')
+
+        url = _helpers.update_query(TOKEN_INFO_URL, query_params)
+
+        response = request(url=url, method='GET')
+
+        return json.loads(response.data.decode('utf-8'))
+
+    yield _token_info
+
+
+def verify_environment():
+    """Checks to make sure that requisite data files are available."""
+    if not os.path.isdir(DATA_DIR):
+        raise EnvironmentError(
+            'In order to run system tests, test data must exist in '
+            'system_tests/data. See CONTRIBUTING.rst for details.')
+
+
+def pytest_configure(config):
+    """Pytest hook that runs before Pytest collects any tests."""
+    verify_environment()