Add admin prototype

issue #2270

- from https://github.com/manuelmitasch/ghost-admin-ember-demo
- Not working properly: added ic-ajax mock in app.js but promise not resolving => loading route always active
Minor refactoring of the routers:

- Moved post route under posts resource
- Changed ajax to use ic.ajax.request instead of ic.ajax.raw
Amend fixtures & put body classes in styleBody mixin

- Change fixture response of posts route to actual format.
- Extracted classNames logic of routes into style-body mixin.
- Additionally replaced all double-quotes with single-quotes for style conformance.