issue 2032 - add modelFor to adapterFor

I think the correct syntax for calling `adapterFor` in @jrhe examples should be `this.store.adapterFor('cat').toString();` instead of `this.store.adapterFor(App.Cat).toString();` and `this.store.adapterFor('dog').toString();` instead of `this.store.adapterFor(App.Dog).toString();`

Updating the syntax will not work without my change to add modelFor to adapterFor, which I believe is needed and should be used.

@igorT, do you agree with that assessment, should modelFor be getting called within adapterFor? 

If that is true and the syntax used by @jrhe was incorrect as I suspect, what should the failure mode be for users that do not pass in a string value to adapterFor?