Replace testling with Karma and SauceLabs

The testling infrastructure is not reliable. We should switch to Karma and SauceLabs to get stable tests and a [meaningful status badge](https://docs.saucelabs.com/reference/status-images/).

This commit changes the test runner for web tests to Karma. Tests are executed in PhantomJS when running `npm test`. @spoike unfortunately I cannot configure the Travis build to use SauceLabs. This task will fall to you :).

Fixes #3