Don't use root logger

[ulope]

If no "Application" logger is available traitlets.log.get_logger() falls back to grabbing the root logger.

This makes it impossible to selectively silence it's debugging output - for example the (IMO quite useless) "Using default logger" message.

To reproduce:

import IPython
import logging

logging.basicConfig(level=logging.DEBUG)
IPython.embed()

[rkern]

Fixed by #233.