Changed relative import for python 3 compatibility.

I found that I could not use this package with python 3 due the following import error.

ImportError: cannot import name HttpNtlmAuth

This change allows it to be imported. I have not confirmed complete success with python 3 yet, this was the first step.
Merge pull request #31 from keith-gray-powereng/patch-1

Changed relative import for python 3 compatibility.