Use Ember.run.later instead of setTimeout in FixtureAdapter
Be asynchronous (though deterministic) when simulateRemoteResponse is false

It's not clear that there is really a use case for synchronous responses
in FixtureAdapter. Asynchronous responses on the other hand are much
preferable for realistic testing.
Merge pull request #744 from CrowdStrike/fixture-adapter

Fixture adapter
Added 'isLoading' flag to DS.State