Minor helper refactor

Hi Liav,

On this branch I:
  * refactored the test helper setup
  * Added `taskcluster.utils.fromNow('1 hour')` to taskcluster client
  * Started using `PulseTestReceiver` exposed as `helper.receiver` to receive messages as a promise. For docs see: https://github.com/taskcluster/taskcluster-base/blob/f4bab11da99771b582ff2833cfe55f2081e054d2/testing.js#L16-L57

I think these things will make it a lot simpler... I also added two very minimalistic tests to see if providing environment variables works with docker-worker. See [docker-worker docs](http://docs.taskcluster.net/docker-worker/) for details on what can be specified in `task.payload`.

Anyways, let me know what you think of this... And we can merge it. After merging it (which includes your PR), we can work on adding more tests and improving existing tests to cover more features.

By the way, I'm going to remove `node_modules/` from `master` right now so we don't have to be annoyed by that anymore :)