TST: add travis configuration file.

Here's something neat. If you add this file into the repo, and hook up Travis-CI (through "settings" => "webhooks and services". Then - whenever we push into github, it will go and download popeye, and run the test suite on a remote machine. See https://travis-ci.org/nipy/dipy for an example of how that works. That way, every push is monitored, and you get immediate feedback on whether or not things are working. Very useful, once the tests are working.