add in a logging NullHandler

Firstly, thanks for the module. This was exactly what I was looking for. However, we're stuck deploying to RHEL6 which ships with python 2.6, and thus I needed to make a small change for compatibility.

Simply, logging.NullHandler was added in python 2.7.

I've added a local NullHandler definition which comes from the documentation at:

https://docs.python.org/2.6/library/logging.html#configuring-logging-for-a-library