Add IAM signer (#108)

diff --git a/google/auth/crypt.py b/google/auth/crypt.py
index 1305cc8..05839b4 100644
--- a/google/auth/crypt.py
+++ b/google/auth/crypt.py
@@ -182,6 +182,7 @@
     def __init__(self, private_key, key_id=None):
         self._key = private_key
         self.key_id = key_id
+        """Optional[str]: The key ID used to identify this private key."""
 
     def sign(self, message):
         """Signs a message.