Add Service and Vhost models, show them on /apps/:app_id show page

@mixonic for you

 * also adds a `{{plural-string singular count}}` helper
 * Needs more permissive CORS from api (https://github.com/aptible/api.aptible.com/pull/122)

fixes #21
Add Operation model, show operations at /apps/:id/operations

Shows operation type, status, created by (name + gravatar), created on
Adds [ember gravatar addon](https://github.com/johnotander/ember-cli-gravatar)

Not styled.

Fixes #24
Merge pull request #22 from bantic/app-show-page-21

Add Service, Vhost models, show them on /apps/:app_id show page, add Operation model, show it at /apps/:id/operations