Add service_account.Credentials.with_claims (#140)

diff --git a/google/oauth2/service_account.py b/google/oauth2/service_account.py
index 58580e2..f8a27bf 100644
--- a/google/oauth2/service_account.py
+++ b/google/oauth2/service_account.py
@@ -70,6 +70,7 @@
 .. _RFC 7523: https://tools.ietf.org/html/rfc7523
 """
 
+import copy
 import datetime
 
 from google.auth import _helpers
@@ -246,6 +247,29 @@
             subject=subject,
             additional_claims=self._additional_claims.copy())
 
+    def with_claims(self, additional_claims):
+        """Returns a copy of these credentials with modified claims.
+
+        Args:
+            additional_claims (Mapping[str, str]): Any additional claims for
+                the JWT payload. This will be merged with the current
+                additional claims.
+
+        Returns:
+            google.auth.service_account.Credentials: A new credentials
+                instance.
+        """
+        new_additional_claims = copy.deepcopy(self._additional_claims)
+        new_additional_claims.update(additional_claims or {})
+
+        return Credentials(
+            self._signer,
+            service_account_email=self._service_account_email,
+            scopes=self._scopes,
+            token_uri=self._token_uri,
+            subject=self._subject,
+            additional_claims=new_additional_claims)
+
     def _make_authorization_grant_assertion(self):
         """Create the OAuth 2.0 assertion.