Add a test watcher task, closes #195

The [preferred method][1] (via Grunt watch) runs extremely slow ([~8
seconds][2]), perhaps related to [karma-runner/grunt-karma#63][3].

Therefore, call Karma directly and add an `npm script` for convenience:

    $ npm run-script test-watch

[1]: https://github.com/karma-runner/grunt-karma#karma-server-with-grunt-watch
[2]: https://gist.github.com/tlvince/9249572#file-gistfile1-txt
[3]: https://github.com/karma-runner/grunt-karma/issues/63