Add public property google.auth.credentials.Signing.signer (#110)

diff --git a/tests/oauth2/test_service_account.py b/tests/oauth2/test_service_account.py
index e6ce631..f40ebf2 100644
--- a/tests/oauth2/test_service_account.py
+++ b/tests/oauth2/test_service_account.py
@@ -134,6 +134,9 @@
         signature = self.credentials.sign_bytes(to_sign)
         assert crypt.verify_signature(to_sign, signature, PUBLIC_CERT_BYTES)
 
+    def test_signer(self):
+        assert isinstance(self.credentials.signer, crypt.Signer)
+
     def test_signer_email(self):
         assert self.credentials.signer_email == self.SERVICE_ACCOUNT_EMAIL
 
diff --git a/tests/test_app_engine.py b/tests/test_app_engine.py
index dd410d9..af60bcf 100644
--- a/tests/test_app_engine.py
+++ b/tests/test_app_engine.py
@@ -139,6 +139,10 @@
         assert signature == mock.sentinel.signature
         app_identity_mock.sign_blob.assert_called_with(to_sign)
 
+    def test_signer(self, app_identity_mock):
+        credentials = app_engine.Credentials()
+        assert isinstance(credentials.signer, app_engine.Signer)
+
     def test_signer_email(self, app_identity_mock):
         credentials = app_engine.Credentials()
         assert credentials.signer_email == credentials.service_account_email
diff --git a/tests/test_jwt.py b/tests/test_jwt.py
index 3959260..e4a9a0a 100644
--- a/tests/test_jwt.py
+++ b/tests/test_jwt.py
@@ -264,6 +264,9 @@
         signature = self.credentials.sign_bytes(to_sign)
         assert crypt.verify_signature(to_sign, signature, PUBLIC_CERT_BYTES)
 
+    def test_signer(self):
+        assert isinstance(self.credentials.signer, crypt.Signer)
+
     def test_signer_email(self):
         assert (self.credentials.signer_email ==
                 SERVICE_ACCOUNT_INFO['client_email'])