Do not make assumptions about features.json.

This removes the automatic assumption by `ember-dev` that if a
features.json exists we should not test all features.

Also, uses the new `ENABLE_OPTIONAL_FEATURES` instead of
`ENABLE_ALL_FEATURES` to ensure that any features explicitly enabled are
still enabled in the tests.
Merge pull request #92 from rjackson/use_undefined_in_features

Do not make assumptions about features.json.