Add support for Tox (http://tox.testrun.org/) and Travis CI (http://about.travis-ci.org/)

These are both testing tools that help you check that your software is working, including with multiple versions of Python, e.g.: Python 2 vs. Python 3, etc.

- [Tox](http://tox.testrun.org/)
- [Travis CI](http://about.travis-ci.org/)

Here's [an example of a passing Travis CI build for my fork of your repo](https://travis-ci.org/msabramo/nose-xcover/builds/5294347).

For instructions on how to enable Travis CI for your own repo (takes less than 2 minutes), see [this page](http://about.travis-ci.org/docs/user/getting-started/).