Lookup the store using store:application instead of store:main

[bmac]

This simplifes the process of registering the store when a user
creates a custom one. This change also fixes deprecations warning
introduces in Ember 1.12. The store service is now injected as a
instanceInitializer.

[igorT]

Can @rwjblue or @stefanpenner review?

[rwjblue]

• store:main will still work, with a deprecation in favor of store:application.
• store:application worked in prior versions

Looks good to me. We should probably update any docs that reference 'app/store.js' in favor of 'app/stores/application.js' (both here and in the guides).

[bmac]

@rwjblue I searched through the guides and the API docs and couldn't find any instance of 'app/store.js' (which should have already given a deprecation message).

app/stores/application.js has been the recommend path for a while, however most of the docs that describe how to do it are still using globals so it looks like App.ApplicationStore.