Add tests

This branch does the following:
- adds tests for every method
- enables `npm test` to run tests
- adds Testling support to package.json and the README: please follow the instructions at https://ci.testling.com/ to add the web hook
- fixes a bunch of methods that were broken, that the tests revealed

I'd love to use this module in a bunch of my own - if you're willing to add me as a contributor to Github and npm, that'd be awesome (I'll still do pull requests on Github).

In addition, if you merge this, please do an npm publish :-) I've already bumped the version in package.json.