Add extra_headers to Rietveld.

rietveld.py

Index: rietveld.py
diff --git a/rietveld.py b/rietveld.py
index b2284bb17e652c046987c8104daf26b24f5b9e60..0cf6ed12f6de3531e9d56112641732e7f6ca60ff 100644
--- a/rietveld.py
+++ b/rietveld.py
@@ -40,14 +40,15 @@ upload.logging.setLevel(logging.WARNING)  # pylint: disable=E1103
 
 class Rietveld(object):
   """Accesses rietveld."""
-  def __init__(self, url, email, password):
+  def __init__(self, url, email, password, extra_headers=None):
     self.issue = None
     self.url = url
     if email and password:
       get_creds = lambda: (email, password)
       self.rpc_server = upload.HttpRpcServer(
             self.url,
-            get_creds)
+            get_creds,
+            extra_headers=extra_headers)
     else:
       self.rpc_server = upload.GetRpcServer(url, email)
     self._xsrf_token = None