Test with all feature flags enabled.

* Adds a new `Ember.ENV` param to force all feature flags on.
* Sets up a new option in QUnit to allow toggling the `ENV.ENABLE_ALL_FEATURES` option.
* Changes the standard test target to test with and without features enabled.
* Pretties up the Extend Prototype configuration in QUnit (giving a nicer display).

I also think we should be testing the individual packages with and without the feature flags so I have submitted https://github.com/emberjs/ember-dev/pull/51 to change `EACH_PACKAGE` to test with feature flags on and off.

This should resolve #3421.

/cc @thomasboyt