Add tox.ini for tox

[msabramo]

This allows doing quick sanity checking to make sure it works on various Python versions.

This depends on #3.

[msabramo]

This allows you to test in multiple Python versions by doing:

$ pip install tox
...
$ tox
...
  py26: commands succeeded
  py27: commands succeeded
  pypy: commands succeeded
  py32: commands succeeded
  py33: commands succeeded
  py34: commands succeeded
  pypy3: commands succeeded
  congratulations :)