Add better tests

[buschtoens]

The README states it quite nicely:

weaksauce test

😄

I also added a lot of new stuff in #8 that is virtually untested. We'll need to improve our testing story.

Requirements:

• test code for:
  • JavaScript
    • native class syntax: class extends { ... }, with Babel 6 workaround
    • Ember object model: EmberObject.extend({ ... })
  • TypeScript
    • native class syntax: class extends { ... }, without Babel 6 workaround
    • Ember object model: EmberObject.extend({ ... }), optional
• assert that @task works without options
• assert that @task works with options and actually applies these modifiers
• assert that @taskGroup works without options
• assert that @taskGroup works with options and actually applies these modifiers
• test that the joint decorators, e.g. @restartableTask and @restartableGroup work as expected
• no need to assert that ember-concurrency works as it should, but can be expected in order to aid in writing tests

Maybe we can do the joint decorators in some sort of generative way in order to reduce the amount of test code we need to maintain.