Deprecate App.Store in favor of App.ApplicationStore.

This makes the application level store lookup much closer to the reset of our conventions (ala `App.ApplicationAdapter` and `App.ApplicationSerializer`).

This change also allows specifying a custom store when using non-global resolver (i.e. EAK/ember-cli). Previously, we were only looking for a property `Store` hung off of the application instance. Now you can have a module named (in the case of stock EAK setup): `app/stores/application` or `app/application/store.js` (pods structure).

A deprecation warning was added, and the prior technique still works so this is not a breaking change (although I believe that we should remove before the prior lookup prior to 1.0).