Add Signing.singer_email (#89)

diff --git a/google/auth/app_engine.py b/google/auth/app_engine.py
index 566475e..608651d 100644
--- a/google/auth/app_engine.py
+++ b/google/auth/app_engine.py
@@ -110,3 +110,8 @@
     @_helpers.copy_docstring(credentials.Signing)
     def sign_bytes(self, message):
         return app_identity.sign_blob(message)
+
+    @property
+    @_helpers.copy_docstring(credentials.Signing)
+    def signer_email(self):
+        return self.service_account_email