Make service_account_email public, read-only (#76)

diff --git a/google/auth/app_engine.py b/google/auth/app_engine.py
index d20ddf6..566475e 100644
--- a/google/auth/app_engine.py
+++ b/google/auth/app_engine.py
@@ -87,6 +87,13 @@
         self.token, self.expiry = token, expiry
 
     @property
+    def service_account_email(self):
+        """The service account email."""
+        if self._service_account_id is None:
+            self._service_account_id = app_identity.get_service_account_name()
+        return self._service_account_id
+
+    @property
     def requires_scopes(self):
         """Checks if the credentials requires scopes.