Refactor to use the extracted lib ember-test-helpers

This PR extracts the core logic from ember-qunit into [switchfly/ember-test-helpers](https://github.com/switchfly/ember-test-helpers). This library now simply wraps ember-test-helpers with QUnit-specific helpers. This approach allows other libraries, such as [ember-mocha](https://github.com/switchfly/ember-mocha), to share the same internals.

This PR also removes the `dist/` dir. @rwjblue favors creating a separate bower repo for distribution.

Also, karma has been replaced with testem to be consistent with ember-cli and its related libraries.

Credit for backing all of this work, including the development of ember-test-helpers and ember-mocha, goes to my client [Switchfly](https://github.com/switchfly). Many thanks to them!