Hellobar integration

Hellobar integration (https://github.com/segmentio/analytics.js-integrations/issues/78).

* No tracking, just loads the script (which appends a bar - a div element - to the DOM).

#### Important
 * `#load should change loaded state` test is failing with `Uncaught Script error. (:0)`. Not sure what to do with that, the scripts seems to be OK, when I load it on my test page in Chrome, there are no errors. And even if there would be errors, it's a 3rd party script so I'm not exactly sure what to do there. I can look into that but I figured maybe one of you guys has seen this before?

#### Less important
 * How do we test integrations that require a valid api key? I've signed up with Hellobar and I'm using the key I got there (which is what e.g. SnapEngage is doing) but that seems brittle (what if they cancel inactive accounts). What's the usual test plan, please?