Allow development only feature flags.

When a flag with `development-only` is found in `features.json` it will
be enabled (flags stripped) for `ember.js`, and disabled (flags
stripped) for `ember.prod.js`.

Conflicts:
	features.json
Fix argument order for handleMandatorySetter.
Use separate FEATURES for dev vs prod builds.
Use `ember-tests.prod.js` when testing prod builds.