Allow parent and child records to be saved in the same commit

Currently (as of 4c47535) you can't create a new parent and child record in the same commit (see #437).

We're hoping to use this pull request as the start of a conversation about how best to fix this issue. We've added a failing integration test for the interaction between the store and REST adapter.

The current error is:

```
<DS.StateManager> could not respond to event didChangeData in state rootState.loaded.created.inFlight
```

This happens because the store, upon receiving a call to `didSaveRecord`, assumes the parent is in the `loaded.saved` state. It's not, however, because it still has a dirty factor for its has-many relationship.