Add fancy locking to oauth2client.

Reviewed in http://codereview.appspot.com/4919049/
diff --git a/oauth2client/file.py b/oauth2client/file.py
index da666c4..b7f9c7d 100644
--- a/oauth2client/file.py
+++ b/oauth2client/file.py
@@ -44,7 +44,7 @@
       f = open(self._filename, 'r')
       credentials = pickle.loads(f.read())
       f.close()
-      credentials.set_store(self.put)
+      credentials.set_store(self)
     except:
       credentials = None
     self._lock.release()