Failure on Windows when getting default user

On windows I get the following error:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "lice\core.py", line 103, in main
    parser.add_argument('-o', '--org', dest='organization', default=guess_organization(),
  File "lice\core.py", line 46, in guess_organization
    org = os.environ["USER"]
  File "C:\Python27\lib\os.py", line 423, in __getitem__
    return self.data[key.upper()]
KeyError: 'USER'

I've tried to fix this by using the [getpass](http://docs.python.org/2/library/getpass.html#getpass.getuser) module instead.