no python2.7 support

[Bonko]

Hi,

the setup.py states that backoff should work with python2.7.

But the used NullHandler was introduced in python 3.1: https://docs.python.org/3.2/library/logging.handlers.html?highlight=nullhandler#logging.NullHandler

The error message when trying to install is:

python2.6 setup.py 
Traceback (most recent call last):
  File "setup.py", line 3, in <module>
    import backoff
  File "/home/bonko/git/backoff/backoff.py", line 104, in <module>
    logger.addHandler(logging.NullHandler())
AttributeError: 'module' object has no attribute 'NullHandler'

[Bonko]

I just found out that it is already existent in python2.7: https://docs.python.org/2/library/logging.handlers.html#nullhandler

Maybe it was removed in 3.0 and therefore the documentation states that it is new in 3.1.

Sorry for the inconvenience.