Add Signing.singer_email (#89)

diff --git a/google/oauth2/service_account.py b/google/oauth2/service_account.py
index 24c852b..48b537d 100644
--- a/google/oauth2/service_account.py
+++ b/google/oauth2/service_account.py
@@ -317,3 +317,8 @@
     @_helpers.copy_docstring(credentials.Signing)
     def sign_bytes(self, message):
         return self._signer.sign(message)
+
+    @property
+    @_helpers.copy_docstring(credentials.Signing)
+    def signer_email(self):
+        return self._service_account_email