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).
Merge pull request #1808 from rjackson/deprecate-store-on-app

Deprecate App.Store in favor of App.ApplicationStore.