Add tox.ini for tox support

I was helping a coworker fix up a pull request where the Python 3.3 tests failed on Travis, and I suggested that he use tox to make sure that the testsuite passed on all of the supported Python versions. I found it a little odd that tox was included in `requirements.txt`, even though there was no `tox.ini`. Here is a simple one, ported from `.travis.yml` and compatible with the version of tox listed in `requirements.txt`.