Add Travis Continuous Integration support

This pull request adds support for the free [Travis CI](https://travis-ci.org/) service. With Travis, every push to this repository will be automatically checked out and tested on different versions of Node.js (currently 0.6, 0.8, and 0.9).

Travis also tests all incoming pull requests, and tells if they pass the tests or not.

To enable Travis support you'll have to (in addition to merging this PR) register on https://travis-ci.org/ and enable Travis for your repository in the [profile page](https://travis-ci.org/profile)

You may also want to include the build status image somewhere in the beginning of your README with Markdown like:

```markdown
[![Build Status](https://travis-ci.org/slightlyoff/cassowary-js-refactor.png?branch=master)](https://travis-ci.org/slightlyoff/cassowary-js-refactor)
```

The Travis build status for my fork can be seen in https://travis-ci.org/bergie/cassowary-js-refactor/builds