travis-ci integration

All tests are passing now, and only a few lines are required to reap the benefits of GitHub/travis-ci integration.

For the indicator button in the `README.md` to work, you will have to login to travis-ci.org with GitHub once, and activate the webhook on netCDF's GitHub page <http://docs.travis-ci.com/user/getting-started/>. Then just update the button with the new links.