configure Travis CI services

This pull request adds `.travis.yml` file containing a configuration for [Travis CI](https://travis-ci.org/) testing of the module.

This configuration is the third step in the process of [getting started](http://about.travis-ci.org/docs/user/getting-started/) with Travis CI. You may be willing to complete that process for your project in order to see (almost immediately) if any of the following commits (and pull requests) pass the tests (`npm test`) properly.

(I have completed that process for my fork and thus you may see the “Good to merge” message from Travis CI below.)

Four test that are known to fail (#136) were ordered to skip (otherwise `npm test` would already fail inevitably, and thus its results would become useless). Now, if `npm test` passes for the result of some commit, it means that at least the number of failing tests is not increased.