Add tox.ini for tox (http://tox.testrun.org/)

[msabramo]

Tox is a tool that lets you easily test across multiple Python versions locally (i.e.: you don't have to push to GitHub to test like you do with Travis CI, which I also like).

I think this would be a good thing to have when adding Python 3 support.

[msabramo]

Sample of use:

marca@marca-mac:~/dev/git-repos/MySQLdb1$ tox -e py26,py27
...
  py26: commands succeeded
  py27: commands succeeded
  congratulations :)