Introduce Testem

Add Testem as an option for running tests. For example,

```JavaScript
testem -l Chrome,Firefox,Safari,PhantomJS
```

Will run tests in all these environments in a continuous-build style. If you connect to the URL in the Testem output you can run the tests in any JavaScript-running browser:

![](https://api.monosnap.com/image/download?id=yQXgZmlphb6jp6oDnFAm0OJKk4fceP)

The existing PhantomJS script has not been removed, since [it runs tests against packages separately instead of together](https://github.com/tildeio/htmlbars/commit/4af5862f27ca453a56e76b9f8f6b592d5e5a255f), something I'm not sure we could easily do with Testem short of changing the QUnit runner.