a work-around to avoid cpython bug (15881)

In python < 2.7.4, a lazy loading of package `pbr` will break setuptools if some other modules registered functions in `atexit`.
solution from: http://bugs.python.org/issue15881#msg170215
Merge pull request #30 from philiptzou/patch-1

a work-around to avoid cpython bug with atexit (15881)