intbitset: add tox for multienv testing

* Adds tox.ini file to allow for easy testing under multiple Python versions
  using virtual envs.

Signed-off-by: Lars Holm Nielsen <lars.holm.nielsen@cern.ch>
intbitset: fix packaging to include C files

* Fixes an issue causing the generated Python package not to
  include the C source files.

Signed-off-by: Lars Holm Nielsen <lars.holm.nielsen@cern.ch>
Merge pull request #1 from lnielsen-cern/tox-packaging

Tox packaging