Add app_engine.Signer (#97)

diff --git a/google/auth/crypt.py b/google/auth/crypt.py
index 8d5ac7c..618afe5 100644
--- a/google/auth/crypt.py
+++ b/google/auth/crypt.py
@@ -186,7 +186,7 @@
             message (Union[str, bytes]): The message to be signed.
 
         Returns:
-            bytes: The signature of the message for the given key.
+            bytes: The signature of the message.
         """
         message = _helpers.to_bytes(message)
         return rsa.pkcs1.sign(message, self._key, 'SHA-256')