Add IAM signer (#108)

diff --git a/tests/test_iam.py b/tests/test_iam.py
new file mode 100644
index 0000000..5ac9911
--- /dev/null
+++ b/tests/test_iam.py
@@ -0,0 +1,100 @@
+# Copyright 2017 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 base64
+import datetime
+import json
+
+import mock
+import pytest
+from six.moves import http_client
+
+from google.auth import exceptions
+from google.auth import iam
+from google.auth import transport
+import google.auth.credentials
+
+
+def make_request(status, data=None):
+    response = mock.Mock(spec=transport.Response)
+    response.status = status
+
+    if data is not None:
+        response.data = json.dumps(data).encode('utf-8')
+
+    return mock.Mock(return_value=response, spec=transport.Request)
+
+
+def make_credentials():
+    class CredentialsImpl(google.auth.credentials.Credentials):
+        def __init__(self):
+            super(CredentialsImpl, self).__init__()
+            self.token = 'token'
+            # Force refresh
+            self.expiry = datetime.datetime.min
+
+        def refresh(self, request):
+            pass
+
+    return CredentialsImpl()
+
+
+class TestSigner(object):
+    def test_constructor(self):
+        request = mock.sentinel.request
+        credentials = mock.Mock(spec=google.auth.credentials.Credentials)
+
+        signer = iam.Signer(
+            request, credentials, mock.sentinel.service_account_email)
+
+        assert signer._request == mock.sentinel.request
+        assert signer._credentials == credentials
+        assert (signer._service_account_email ==
+                mock.sentinel.service_account_email)
+
+    def test_key_id(self):
+        key_id = '123'
+        request = make_request(http_client.OK, data={'keyId': key_id})
+        credentials = make_credentials()
+
+        signer = iam.Signer(
+            request, credentials, mock.sentinel.service_account_email)
+
+        assert signer.key_id == '123'
+        auth_header = request.call_args[1]['headers']['authorization']
+        assert auth_header == 'Bearer token'
+
+    def test_sign_bytes(self):
+        signature = b'DEADBEEF'
+        encoded_signature = base64.b64encode(signature).decode('utf-8')
+        request = make_request(
+            http_client.OK, data={'signature': encoded_signature})
+        credentials = make_credentials()
+
+        signer = iam.Signer(
+            request, credentials, mock.sentinel.service_account_email)
+
+        returned_signature = signer.sign('123')
+
+        assert returned_signature == signature
+
+    def test_sign_bytes_failure(self):
+        request = make_request(http_client.UNAUTHORIZED)
+        credentials = make_credentials()
+
+        signer = iam.Signer(
+            request, credentials, mock.sentinel.service_account_email)
+
+        with pytest.raises(exceptions.TransportError):
+            signer.sign('123')