Ember.js: Add assets img + css (ember-hacks.css) to ember admin

Closes #2276

- Adds the images to `core/client/assets/img`
- Adds css with ember hacks to `core/client/assets/css`
- Configures middleware to provide assets as static files at url `/ghost/ember`
- Adds ember option to assets helper
- Modifies default-ember.hbs to use ember option on asset helper
- Remove inline style definition in editor.hbs
- Modifies .gitignore to include ember-hacks.css (in ignored `core/client/assets/css` folder)