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

diff --git a/google/oauth2/service_account.py b/google/oauth2/service_account.py
index 48b537d..b71b8ac 100644
--- a/google/oauth2/service_account.py
+++ b/google/oauth2/service_account.py
@@ -320,5 +320,10 @@
 
     @property
     @_helpers.copy_docstring(credentials.Signing)
+    def signer(self):
+        return self._signer
+
+    @property
+    @_helpers.copy_docstring(credentials.Signing)
     def signer_email(self):
         return self._service_account_email