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

diff --git a/google/auth/jwt.py b/google/auth/jwt.py
index dfaf2e6..418be6b 100644
--- a/google/auth/jwt.py
+++ b/google/auth/jwt.py
@@ -468,6 +468,11 @@
     def signer_email(self):
         return self._issuer
 
+    @property
+    @_helpers.copy_docstring(credentials.Signing)
+    def signer(self):
+        return self._signer
+
     def before_request(self, request, method, url, headers):
         """Performs credential-specific before request logic.