Add support for tox (http://tox.testrun.org/) and Travis CI (http://travis-ci.org/)
These are testing tools.

Tox is run locally and can be used on code that is not even checked in. It runs the test suite on various Python versions.

Travis CI is a hosted CI system that can run tests whenever new commits are pushed to the GitHub repo.

You can see the Travis CI status for my fork at http://travis-ci.org/#!/msabramo/pandas -- it's failing at the moment -- it probably needs some tweaks as I am completely new to pandas and I'm probably not building it correctly.
