Completed Python 3.x support

* Completes support for Python 3.x, while retaining
  back-ward compatibility for Python 2.5 onwards
  by using the six module. (closes #3)

Signed-off-by: Samuele Kaplun <samuele.kaplun@cern.ch>
intbitset v2.1

Signed-off-by: Samuele Kaplun <samuele.kaplun@cern.ch>