default application logger shouldn't propagate

avoids annoying duplicate messages on the root logger under some circumstances.

If apps want to hook up their own logging, they should assign the log attribute.

IPython/config/application.py

@@ -146,6 +146,7 b' class Application(SingletonConfigurable):'
         """
         log = logging.getLogger(self.__class__.__name__)
         log.setLevel(self.log_level)
+        log.propagate = False
         _log = log # copied from Logger.hasHandlers() (new in Python 3.2)
         while _log:
             if _log.handlers: